There are 50 gold film reels in the game. None of them is missable and all gold film reels are immediately save after you pick it up. I recommend to collect the gold film reels in free roam mode, you can enter this mode after you complete a desk (except patrol) from the main menu. Select cases then a complete desk and scroll down to "The streets of L.A.." This is the free roam mode.
Timeline:
*** Spoiler - click to reveal ***00:08 - #1 (The Big Sleep)
00:38 - #2 (The Letter)
01:18 - #3 (Brute Force)
01:55 - #4 (Nightmare Alley)
02:33 - #5 (Where the Sidewalk Ends)
03:07 - #6 (Gilda)
03:42 - #7 (In a Lonely Place)
04:17 - #8 (Odd Man Out)
04:53 - #9 (The Night of the Hunter)
05:26 - #10 (The Spiral Staircase)
05:58 - #11 (The Woman in the Window)
06:29 - #12 (Sunset Boulevard)
06:58 - #13 (The Narrow Margin)
07:36 - #14 (White Heat)
08:17 - #15 (Crossfire)
08:50 - #16 (M)
09:22 - #17 (Thieves' Highway)
09:53 - #18 (The Killers)
10:32 - #19 (The Lady from Shanghai)
11:03 - #20 (The Third Man)
11:34 - #21 (Shadow of a Doubt)
12:11 - #22 (Laura)
12:42 - #23 (The Set-Up)
13:23 - #24 (The Killing)
13:56 - #25 (Night and the City)
14:36 - #26 (The Big Clock)
15:09 - #27 (The Naked City)
15:41 - #28 (This Gun for Hire)
16:28 - #29 (Sweet Smell of Success)
17:03 - #30 (Rififi)
17:34 - #31 (Murder, My Sweet)
18:08 - #32 (The Big Carnival)
18:42 - #33 (Out of the Past)
19:15 - #34 (The Asphalt Jungle)
19:53 - #35 (Touch of Evil)
20:57 - #36 (Mildred Pierce)
22:12 - #37 (The Big Heat)
22:46 - #38 (Angels with Dirty Faces)
23:33 - #39 (Strangers on a Train)
24:12 - #40 (The Maltese Falcon)
24:41 - #41 (Leave Her to Heaven)
25:18 - #42 (Key Largo)
25:52 - #43 (Pickup on South Street)
26:28 - #44 (House of Bamboo)
27:11 - #45 (Gun Crazy)
27:46 - #46 (Scarlet Street)
28:21 - #47 (Detour)
28:53 - #48 (Body and Soul)
29:44 - #49 (Double Indemnity)
30:15 - #50 (Notorious)
If you want you can use the map and the tracker from Rockstar's socialclub.
https://socialclub.rockstargames.com/games/lan/checklistUnder "Film Reels " you can see how many and which gold film reels you have already collected (map). The map from L.A. shows you also the locations but for many collectibles the map is really inaccurate. You should use the map then rather as a clue.
A Note, I collect in my video from north to south.