Right or reversed tire?
 

I just got a 2001 GT Avalanche 1.0. I realised that the rear tire seems to be installed in the opposite direction. I scanned through the tire but cannot find the direction sign anywhere, so is this done on purpose to increase grip or did the shop install it wrongly?

It doesent matter what way the tire goes on, it will probably wear a little faster if its installed on backwards though.

If there are no directional arrows on the tires there should not be any problems. Depending on the tread pattern, some tires are mounted differently from front to back to give better traction on the front when breaking and on the rear for better traction while accelerating or climbing.