To cut to the chase, in utime on Garry's Mod I have 2 weeks and 1 day of playtime total. Combining every username i've used on Steam, I only have about 120 hours on Gametracker. Is this common, and are there any ways to fix this?

Utime, as I understand it, is installed for a server, and is connected directly to your game.
This is fine for monitoring one server, perhaps even two.

Gametracker scans 155000+ servers, and 98 game types at approximately 4 minutes intervals.
The time between scans naturally fluctuates between 3 and 8 minutes in normal operation.

It would not be practicably possible to maintain full time connections to that many clients for constant real time monitoring.

It is normal.
It can't be fixed because it's not broken. It is what it is.