What is the cheapest gmod server hosting site?

I'd recommend starting of self-hosting on your pc as it's free and a good way to learn a thing or two. Finding a host after you've run one locally will offer protection from DDoS attacks for only a few bucks and also enable you to run the server more reliably 24/7. I currently own one called Hexane Networks which has very positive feedback from its clients.

https://www.hexanenetworks.com