Well, GURPS doesn't have to be played in a realistic mode. But in general, tactics that work well in Real World combat work well in GURPS, too - and something that would be a very stupid idea in Real Life is probably difficult to pull off in GURPS as well. This is more difficult to do in d20...
I've GMed GURPS in fantasy campaigns, and in general I'd say that as long the characters are taking the proper precautions, GURPS combat isn't that much more lethal than mid-level D&D. On the other hand, if the PCs get into combat while not wearing any armor, or charging into the midst of a few dozen goblins, then it's not too surprising if they get hurt.
Wear armor, devise some tactics, and watch your back - and you will last for a long time in GURPS...