Man, I haven't played GURPS in decades. Excuse me while I dust off my copy of GURPS 3E (literally, I had to brush dust off the cover) and refresh my memory. Let's see here...
So. If you haven't played any edition of D&D since the '90s, there are several things that you'll want to keep in mind as you transition to D&D in general and 5E in particular:
D&D is combat-heavy. GURPS treats combat as one element of the game among many; it's not unusual that some PCs will be built for combat and others will focus elsewhere. In D&D, combat is the beating heart of the game. All D&D characters are combat-capable unless players go out of their way to cripple themselves, and the rules are built for adventures in hostile territory where enemies are everywhere and frequent battles are inevitable. You don't have to run it that way, but the game strongly encourages it.
D&D is cinematic. GURPS tries for at least a modicum of realism. D&D (especially 4E and 5E) favors streamlined mechanics and cinematic action. This is necessary both for the characters (so they can sustain the damage of so many battles) and players (so battles can be resolved quickly). Trying to make D&D more realistic is a common pastime, but I've never seen it work out very well; again, you are fighting the game. It will go a lot smoother if you embrace the action. And for the love of Gygax, do not inquire into what exactly is represented by "hit points." That way lies madness.
The D&D power curve is insane. GURPS characters gain power fairly slowly, and face diminishing returns that encourage breadth over depth. D&D characters rocket from peasant to demigod at a blistering pace, faster by far than any other RPG I have ever played. You can adjust this a bit by controlling how much XP you award (milestone XP is your friend), but players get unhappy if they never get to play with new toys, and they only get new toys by climbing the power curve. Especially at high levels, those toys will take your carefully planned adventure and knock it into a cocked hat. I strongly recommend starting at a low level so you can at least get your feet under you.
D&D provides a built-in framework for the campaign. All of the above may sound rather limiting compared to the free-form approach of GURPS. And it is. But it also means a lot of the work is done for you. Players are supplied with prefab archetypes in the form of character classes. They can be embellished with background details and customization, but they also work fine as-is. The DM has a standard narrative ready for any adventure: "Go to this place, explore it, kill the monsters there, and take the treasure." You don't have to follow that formula every time, but it's always available when you need it; you have only to deploy it, and the machinery of the game whirs into motion to help entertain you and your players.