Finally i got to Lvl 20 to use the Grav Gun, which seems to be the only weapon i really enjoy (i dont like the need of lvl 20 to use it *gg*)
I got a Grav Gun with the lvl-up and equipped it. The gun is not to be seen on my Character, but is there in the Inventory Screen (so not like many others have the Problem with equippment disappearing). Not on the Modell nor to be seen at missions. But it is, mostly, working. Then, sometimes, it stops shooting (not a overheat problem) and i have to change my Weapon for the second load, and switch back to have it work again.
And it is not the Grav Gun that i became at hitting lvl 20, i got another drop, and even crafted one (with over 300 minutes of craft Time) and the same problems with the others, so maybe its a general Grav Gun Issue.