Here is the Gunsmiths building map layout and the chalk drawing of the guns and weapons that are inside. There are 5 weapons you can draw which are the PDW57, AN94, Galvaknuckles, AK74U and the SVU-AS sniper rifle. Each time you draw a weapon on the wall you earn 1000 credits.
If your low on cash for something or need random crate fast, draw the weapons to get a go in the random crate ready and paid for straight away.
There is also a part for the guillotine hidden in the corner here in the room where the tunnel underneath leads to the bank. Upstairs is a random crate spawn location and access to the tunnel system.
Here is the map below: