The first step is to determine why you have a gap problem. It has been reported here and it has been my experience that doors and lids can be interchanged between cars w/o much difficulty. In other words, if one doesn't fit right, the next one will probably be the same.

If you must, you can weld onto the edge and grind it straight. Bondo on the edge doesn't work and it's not much better to build up on the gutters or jambs. In the rear, the problem is going to be with the tub and in the front it's going to be in the mounting of the fenders. Or, worst yet, the front chassis snout. It's actually kind of neat how the various front half parts relate to each other and the tub.

Thanks Zeke,

My situation is more related to fitting fiberglass fenders, hood, quarters, etc...

I did not know what was the best way to close any gap between fiberglass parts.

For fiberglass you'd want to continue the integrity of the door onto a new edge. In other words, add to the edge with FG tape and resin from an inch or two in.

To insure it's flush you may have to sand a "step" to lay your work in or feather the new edge work into the door.

If you merely use Bondo on the edge it will knock off easily.

doors to fenders - 4mm
fenders to front bonnet - 3mm
I'm at the same stage!