Maintaining your weapons is one of the most important things that you need to look after within the game. Letting it degrade basically means that it’s going to get to a point where you’re firing bubbles and it’s going to take a lot of bullets to kill someone.

First thing you’re going to need is some gun oil in your inventory, this is as far as I can tell the only way to clean up your gun.

When you open the weapon wheel and hover over the weapon you want to clean up, press down on the right stick (R3) and this will bring up the weapons stats and also show you how degraded the weapon is.
You can tell how degraded it is by how much red is showing in each section. If there is a fair amount of red line showing, I would highly recommend cleaning the weapon, which you can do by pressing and holding the Square/B button.

The process will automatically stop once the gun is clean again.

I would advise to check out gunsmiths rather than picking up off the floor as you may be able to find a decent gun with full stats. The ones you pick up off enemies will most likely be worn a fair bit.
If you don’t have any gun oil then you can go and visit a gunsmith for all your gun needs!