Concepts of GURPS 4E vs. D&D 5E

The basic concepts are generally the same as any roleplaying game - tell the GM what you want to do and they tell you what happens or what you have to roll.

General
Anytime you try and do something that requires a roll you do not roll against a DC with modifiers for how good you are, but instead roll against your skill score and try to get lower than it (and your skill goes up as you improve it!).

• Everything is rolled using 3d6, except damage, which is rolled in various numbers of d6's.

Combat
Doing something in Combat is a Maneuver - there are no Actions, Bonus Actions, Movement etc.

• There are maneuvers for attacking, attacking and moving, just moving, and other special things, but your turn is entirely spent with a maneuver of some type.

Each round takes 1 second, so things are close and tense!

Defense is super cool as you can choose to Dodge, Block or Parry nearly any attack!

Weapons are way more unique and have a much greater variety than D&D 5E, and making your own unique weapon is also easy!

If you love the detail, you can aim for certain body parts at different difficulties for different realistic effects!

Movement is a little funky, as you are in 1 second rounds so it does its best to factor in momentum and directions of facing - see Move, Facing & Visibility for more in depth look.

Hit Points and Fatigue Points are super cool: HP doesn't usually restore unnaturally quick (unless you have magic) and Fatigue does a great job of simulating how tired out you get from things.

• Magic (and some abilities) can draw from either one, making for tough decisions and cool mechanics.
• Don’t worry, you usually still can survive if you fall unconscious due to running out of HP, and if you are bad ass/lucky you can still keep fighting when you shouldn't be able to!

Game Master's Toolbox
This game on the GM's side is essentially a giant Toolbox with parts and frameworks for making nearly anything - homebrewing is much easier (you have all the component pieces to work with) and more easily balanced!