I've been playing GURPS for a bit under a decade, and GMing it for about 7 years. Before playing GURPS, I played 4E D&D and a little bit of pathfinder.

Technically speaking, each turn is one maneuver, and each maneuver can have multiple actions. For example, pretty much every maneuver allows you to take one free step in addition to whatever else you're doing. There's also the Move And Attack and Heroic Charge Maneuvers for melee attacks. The difference between the two is that Move and Attack has a penalty to the attack equal to the number of yards moved, while heroic charge forgoes the penalty in exchange for costing 1 FP and is part of the optional Extra Effort rules.

In terms of people who are used to systems with more actions per turn, in my experience, they stumble a bit with it in the very beginning, but generally adapt after a few sessions. It's not all bad, as while you theoretically have less to do on one turn, you also get to go more often, and combats go quickly. I've had a number of sessions in which we got through multiple complete combat encounters in one 3-hour session, along with some roleplaying and planning in-between (most of my sessions are 1-2 combat encounters with the rest of the time being roleplaying). If you want more to do in each turn or have more options, the Martial Arts book is really fantastic and has a lot of optional rules and maneuvers. You can also do a lot just with deceptive attacks (taking penalties to your own attacks to give your opponent a penalty to their defense), feints (if you win a quick contest of weapon skills, your foe gets a big penalty to defense), all-out-attacks, and all-out-defense.

I can't speak specifically to gurps but as a general rule, for any game, I highly recommend playing it as intended for a bit before doing substantial tinkering. To do otherwise is a lot of hubris.