How can I get GP.

You get a total of about 100k GP as you rank up through the first few levels. For more cash, you can save money on repairs by using your M16 and M700 and not paying the repair costs, this will get you about 20-30k more GP if you break them fully. Also, if you buy ZP you can get a lot of GP very very quickly... If you buy 10k ZP for 10 dollars, buying crates gets you about 40k ZP but possibly a lot more, as well as some temporary guns and possibly a nice rare permanent one. If you buy straight GP with your ZP, you can get about 70K but nothing else.

1. Break your M16 and your M700.
I don't necessarily mean go to S&D and break them. S&D USUALLY GIVES YOU THE MOST GP IN MOST GAMES, however I wouldn't necessarily say you should break them that fast. The M16 is pretty decent if you know how to use it... Then again most people here just spray, so you should break it.
2. Don't try to use multiple weapons in games
More Weapons you use = More GP you need to repair each weapon, so I don't recommend buying more weapon bags.
3. Earn ZP.
I won't go into much detail for this part, but I'm sure there was a thread earlier in one of these sub-forums that explains how to earn ZP, but it mainly depends where you live. One just key advice would be to make a fake E-Mail Account.
4. Depends on how well you play
If you're good at this game earning at least a 1.5+ k/d per game, you don't need to do much. If you're average, getting a 1+ k/d per game, you should probably play more, and if you're good at all, getting a negative k/d per game, you should play in a lot of knife battles.
5. DO THE MISSION.
It's very useful. Each day you can get 500 GP. It really goes well long-term wise. Aside from that, if you get the dog tags perfectly, then you can get ZP crates which can either give you more Black market items that you can use, And even more common, GP which you can use for later.

do missions and break the rewards they will give you (xm1014 A and open DE scope crates after that)