The ARMAX has a code that sprays the tags for you and thus, gives you desert eagles, mp5's & knives. If you want to spray them manually, there are plenty of tutorials on Youtube as already pointed out. The problem with desert eagles & mp5's is Groves aiming fails whereas Sindaccos & FBI have better aim with said weapons. Ric-013s's post details how to make your own gang weapons codes. I recommend the spaz12 (to replace the desert eagle), the minigun (to replace the mp5) and the chainsaw to replace the knife because I'm a big fan of the Texas chainsaw massacre movies.
I wanna feel proud of myself for spraying all of them manually, I've never done it before..seriously--It's like the GTA 3--I never beat that either..I also wanna do the last Zero mission so I can get that asset..
@BabeRuth: Oh yeah. You won't need to remind me anymore. Then again, this post is in "cheats & tricks" so I tend to assume. But thanks again for the Spaz12 suggestion.
@Radoonhagaydoon: I've actually sprayed the tags manually but others are lazy. The only things I cheat on are things that just can't be done without cheating/coding. Missions and side missions can, so I don't cheat on those. I myself have never beat GTA 3. Then again never played it, but should. If you can beat Zero's 2nd mission, you can beat the 3rd which is much easier than the 2nd.
You can do them at anytime after they are unlocked as well as heist missions, Wang cars asset missions you can even do them after End of the Line. They are not necessary to make further progress in storyline.