can't get gmod to use more than 2GB RAM

hello guys,
i’m on kubuntu 17.04 now and my specs are good enough for everything and not that neccessary (i have 16GB RAM).
So, i tried every possible start option, but as soon as i start a singleplayer game and the used RAM by gmod goes over about 2.2GB it just crashes saying out of memory.
i know 32bit programs can only use 4GB RAM, but that would be enough. Currently it’s just 2GB.
Please help, airzocker