The small house is real, located at 11901 West Trail Road, in Sylmar.

Despite its small size (just 264 square feet), the house (built in 1940) still stands (as of 2020).
( But it might not be there for long. The land is on the market, with the house described
by Zillow as "in tear-down condition". )

  But ironically, the larger, Spanish-style home you see in this scene, on the hilltop
in the distance, across the canyon is fake - a CGI addition.

Here is a matching Google StreetView of the house and surrounding hills.

         

( Thanks to Michael J for letting me know about this location.)

0:53:10:  The night scene of  the Squad shooting some bad guys & shoving them down a hillside,
was shot in L.A.'s Griffith Park, along a stretch of the winding Vista Del Valle.

Griffith Park is the largest urban park in the United States, containing over 4,300 acres.
It's part of the east end of the Santa Monica mountains. Much of it is rugged, natural
land of hills & canyons, but the park also contains the L.A. Zoo, the Greek Theatre,
the Observatory, Gene Autry Museum, golf courses,a merry go round,
Traveltown Railroad Museum, and the L.A. Equestrian Center.

The official address of the park is 4730 Crystal Springs Drive, but that will
only take you to the southeast corner of the park.  These scenes were
shot on the winding roads in the hills west of the merry go round.
See the map links below for a more specific location:

         

59:28: The scene where the Squad rousts one of Cohen's men from a car, beats him up, and sets fire to his car,
was shot in downtown Los Angeles, on the 500 block of S. Santa Fe Avenue, with the camera looking north
 at the 4th Place bridge ( a small offshoot of the large 4th Street Bridge) as it arches over Santa Fe Ave,
and then over the railroad tracks and the Los Angeles River, to the east.

But they seem to have exaggerated the proximity of City Hall, seen in the background.
It's actually a mile away, to the northwest, and not really visible from this spot.

Here is a matching Google StreetView of the street & bridge.

         

1:13:38: Jerry & Grace are at a rustic-themed restaurant when a pair of Mickey Cohen's
men show up, and try to throw acid in Grace's face.  Jerry stops them.

This scene was shot inside another L.A. landmark: Clifton's Cafeteria, at 648 S. Broadway, in downtown.

And yes, that elaborate redwood forest ambiance is real - Clifton's was (and is) something special.
Besides the redwood trees, the restaurant interior also features a waterfall, fake deer & bison, and a brook.
Built long before Disneyland, it's said that Walt Disney got a few of his ideas from Clifton's.

Clifton's opened at the height of the Great Depression, in 1935. Since then, it has served 170 million guests.
The founder, Clifford Clinton, had a "Golden Rule" policy of letting people pay what they could afford.
Its once-popular Broadway neighborhood fell into disrepair in recent decades, but like the rest of
downtown L.A., Clifton's is making a comeback.  It closed temporarily for renovation in 2011 and
should reopen soon - complete with a restored redwood forest.

Here is a Google StreetView panorama of Clifton's (prior to its renovation).

         

1:16:16:  Not surprisingly, the big ambush in Chinatown was filmed in L.A.'s Chinatown, the touristy
Central Plaza district, which lies between Broadway & Main Street, north of College Street.

One gift shop glimpsed often during the scene is "Sincere Imports", located at
483 Gin Ling Way (near the Hill Street Chinatown gate).

The exploding truck was parked in front of a shop called "Realm" at
425 Gin Ling Way (closer to the Broadway gate), in Chinatown.

Here is a Google StreetView panorama of the Hill Street Gate.

         


[ A note of interest: Originally, the film featured a scene filmed at Grauman's Chinese Theatre,
where gunmen fired through the movie screen into the audience.  However, before the film
could open, something similar happened in real life, when a gunman opened fire at a
movie theatre in Aurora, Colorado.  As a result of that tragedy, they postponed the
release date of "Gangster Squad", removed the Grauman's shootout scene,
and replaced it with this shootout filmed in Chinatown. ]







1:32:00:  As the Squad assembles at night for their final assault on Cohen's gang,
we see a familiar sight behind them - the tower of Los Angeles City Hall.

You'll find that famous tower at 200 N. Spring Street, between 1st Street & Temple Street.

Here is a Google StreetView of City Hall.

         

1:32:41:  Mickey Cohen and his gang are holed up inside a large hotel.
Cohen has rented out all the rooms, and turned the place into a veritable fortress.

It's here that the Gangster Squad mounts its final assault on Cohen's forces,
resulting in a massive gunfight that begins on the front steps, moves in the lobby
(decorated for Christmas) and eventually spills out onto the street and into a nearby park.

In reality, this is the Park Plaza Hotel, at 607 S. Park View Street (at 6th Street),
two miles west of downtown L.A., overlooking MacArthur Park.

The spectacular building (its exterior complete with angels, towering columns and
Egyptian glyphs), was built in 1925 as an Elks lodge, and later became a luxury hotel.
(You can still see a bronze elk head atop the arching windows above the front entrance.)

The neighborhood deteriorated badly over the years, but the landmark building survived.
It is now used mainly as a filming location, and for special events such as weddings.

Both the interior and exterior of the building were used extensively in this shootout scene.

In the photo above, we're looking at the hotel's main entrance on Park View Street.
In the photo below, the scene is inside, shot in the hotel's lobby.

Here is a Google StreetView of the hotel.

         

1:30:44:  Eventually, as casualties mount and the shootout winds down, Mickey Cohen makes a run for it.

As Cohen flees in his car, O'Hara jumps on board, and the car spins out of control,
veering across the lawn of a nearby park, and crashing into a large fountain.

A mano-a-mano fist fight then breaks out between O'Hara and Cohen.

The Park View Hotel got its name because it looks out over MacArthur Park, so you won't be
surprised to learn that this fight scene was filmed in MacArthur Park, at 2230 W. 6th Street.

Don't be fooled by the big fountain.  It's a fake, a prop brought in by the producers.
(In real life, you'll have to settle for the towering fountain in the middle of the park's big lake.)

Here is a Google StreetView panorama of the park.

         

1:43:31:  And that final shot of O'Hara on the beach with his wife & child?

Like most beach scenes in Hollywood films, it was shot in Malibu.

To be more specific, on Westward Beach, near Point Dume (near Tower 1).