Hexagonal grids are at the same point a strength and a weakness in the Gurps System. They are better to represent natural features and give more accurate measurements, but at same time they are much more difficult to draw. The majority of RPGs uses square grids and there are a multitude of ready to use material with square grids in them.
So why not to take advantage of this material? What changes must be made to the combat system for it to work in a square grid?
First, orientation. Now the character has two more sides, having three front squares, four lateral squares, two to each side, and a back square. He can face any of the eight squares that are adjacent or diagonal to him.
Waht about melee weapons reach? We have weapons with reachs of C, 1, 2 and 3. For reachs C and 1 the figures above are enough. For weapons with reach 2 and 3 we can expand the figures above to show which squares can be reached:
The vision arc isn't exactly simple. While in a hexagonal grid arcs of 180° and 240° are trivial to define, in a square grid a good approach would be this:
In the case of ranged weapons crucial differences start to appear. The biggest advantage of hexagonal grids is that to measure the distance between two hexes you cant just count the hexes between them, with minimal errors. While in a square grid counting squares diagonally you obtain a result that is 40% bigger (1.4142... bigger to be more exact) than counting squares in line. To solve this problems you should count each diagonal distance as 1.5 yards. The same is true to movement points, each diagonal movement costs 1.5 movement points.
D&D maps, and most other RPG systems, use a scale in which each square equals 1.5 yards. As the Gurps hex measures 1 yard you must pay attention to this when using D&D maps. Particularly, don't care about the difference. Who has ever heard about subterranean dungeons with spacious corridors 1,5 yards wide?