There are a total of 50 Gold Pieces to collect.
All of the Gold Pieces can be found in the Übercommander Missions (the videos were recorded in those missions). I recommend grabbing all collectibles after beating the main story and unlocking all of the Contraptions.
Some Gold Pieces are only accessible during the Übercommander Missions and are labeled appropriately below.
Manhattan - 18 Gold Pieces*** Spoiler - click to reveal ***
► Harbor District
Dock:
0:40 - Gold 1
1:11 - Gold 2
Hotel:
2:08 - Gold 3
2:28 - Gold 4
5:19 - Gold 5
Police Station:
6:53 - Gold 6
Subway Entrance:
8:37 - Gold 7 [Übercommander Mission]
► Subway District
Main Tunnels:
10:21 - Gold 8
Side Tunnels:
12:08 - Gold 9 [Übercommander Mission]
► Ruins District
Shops Near State Building:
15:31 - Gold 11
16:50 - Gold 12
Multi-Floor Ruins:
18:00 - Gold 10
Subway Entrance:
19:14 - Gold 13 [Übercommander Mission]
► Penthouse District
Main Floor:
21:54 - Gold 14
► Nuclear Bunker District
Command Center:
24:52 - Gold 30
4-Way Intersection:
25:37 - Gold 29
26:29 - Gold 28
Bunker Entrance:
28:57 - Gold 27
Roswell - 10 Gold Pieces*** Spoiler - click to reveal ***
► Downtown District
Exclusive to the Übercommander Mission:
3:10 - Gold 15
5:52 - Gold 16
6:50 - Gold 17
7:33 - Gold 18
► Underground District
Smaller Station:
9:11 - Gold 22
11:52 - Gold 23
Traincars:
13:21 - Gold 24 [Übercommander Mission]
Large Station:
13:41 - Gold 20
14:25 - Gold 21
16:30 - Gold 19
Mesquite, TX - 2 Gold Pieces
New Orleans - 10 Gold Pieces*** Spoiler - click to reveal ***
► Wall District
Outside the Wall / Alligator Farm:
1:44 - Gold 31
► Ghetto District
Main Street:
5:23 - Gold 32
Shantytown:
11:14 - Gold 33
11:52 - Gold 34
Street in Front of Courthouse:
13:02 - Gold 35
► Bienville Street District
Sewers:
14:17 - Gold 37
Main Street:
19:52 - Gold 36
► Lakeview District
Broken Street and Mansion:
21:21 - Gold 40
22:06 - Gold 39
23:41 - Gold 38
Venus - 8 Gold Pieces*** Spoiler - click to reveal ***
► Habitat District
Main Central Hallway:
2:28 - Gold 42 [Übercommander Mission]
Living Quarters:
2:57 - Gold 41
► Transporthalle District
Garage / Tunnel:
5:51 - Gold 43
6:15 - Gold 47 [Übercommander Mission]
► Oberkommando District
Main Control Room:
9:00 - Gold 46
10:36 - Gold 48 [Übercommander Mission]
Entrance / Living Quarters:
11:50 - Gold 45
12:41 - Gold 44
Riverside - 2 Gold Pieces(This area is unlocked after completing all 15 of the Übercommander Missions)