Order a kimsufi ks-3 for ~$20/mo. Garrysmod absolutely requires DDoS protection. You really need a way to block spoofed UDP floods that cause SRCDS to choke as well, I like to run pfsense in front of the gmod servers as well.
There was a time when Softlayer had entry level servers with competitive pricing. Unfortunately, those days are long gone. I'd consider them if AWS was similarly priced with more redundancy.
Raspberry pis are toys, I was given one and I may use it for a digital systems class I need to take. I've deployed about ten of them for use as controllers for crypto mining setups.