Intro

GURPS 4th Edition CAN BE a very tactical game, even on simple fantasy settings with low power levels, player characters have many options on how to attack, how to defend, how to move and even how to dogde, and they still face many more different ways to get killed if they don't "get" the big picture of the situation.

Therefore, having a platform like Roll20 will help a lot players and game masters that like to keep their encounters very tactical, giving players a hand to figure where is cover, who has line of sight, etc.

however, remember to keep your game in a level of complexity that everybody will enjoy, not everyone likes spending an day to figure out if a sniper shot will hit.

Setting the Battlefield

So, lets make your battlefield more GURPS oriented.

Open your page menu and set it like so:

Page size: any by any Scale: 1 hex should be 1 yard or 1 meter, it's not canonical though, feel free to amp this IF (and only IF) everyone is a giant or piloting a MECHA or something.

background: I like to keep it a simple white.
Grid: Enabled, size 1 unit.
Diagonals: Pathfinder gives the best results, where every adjacent hex's center is 1 unit away.
Type: Hex (H) works best.

Show labels is kinda ugly, but in situations where player characters are hidden, it's good to have some feed back like "I am moving to Hex V18".

Opacity: full on! Fog of war: Your mileage may vary, but for me it adds another layer of complexity to something pretty complex. so I prefer to turn it off.
GM opacity: full on!

Macros

At the time of this writing, the dice and math mechanics available for common free users don't give GURPS players any special kind of optimization, since most rolls are under a constant change of bonus and penalties, so there is really one thing you can do: set a macro to roll three six-sided dice (3D in GURPS), the most common roll.

just get a "/r 3d6" macro and you are all set.