Yeah, and your RAM amount? HDD or SSD? how much space is left on those discs? Are you running anything next to it e.g. steam overlay, other things on the background? Do u have a lot of subscription in your workshop for gmod?
A download/upload speed doesnt state consistency, its a point in time, if you get random internet spikes where the connection drops its not visible in such a test unless it happens at exactly that time. But usually with those speeds your internet (should be) stable.