How do we get a gun for free

You just need to play the game, guns are rewarded by getting kills, contesting hills... You can keep track of how much you have to to by going into both the achievements and the equipment tab.
I'll have to warn you, after having 4 guns for each class, it's really hard and slow to get guns, you need a huge amount of kills for the last 3 guns, so patience is something you'll need if you want all the guns...

There is a progressive bar, indicating how much EXP you need/already have for a gun. Global EXP are usually for your online profile and your secondary weapon (Handguns). The other types of EXP are for the other classes. The more EXP you have for the type of class, the more guns you unlock for that class. Usually, after you unlock the first 2 weapons (not including the starting gun), it will take a huge amount of determination and patience to eventually unlock the next. If you are just surfing the web, and heard how great it is, I recommend it to you.
9.5/10
A happy Triggerfist Addict

You have to have in app purchases on. Once in game, swipe to the farthest unlockable (P90, Mini Gun or Beowulf .50). Then press Get Now, and the second you press it, swipe out of the game. Then if done correctly you’ll see the confirmation tab then click ‘cancel.’ Then close the game and reopen it. Then you should see all weapons unlocked for the class. This can also be done with secondaries, perks, and the expansion pack. And you will instantly be level 43.