Gap filling is cosmetic. If you have a gap, it will get filled with epoxy when you glass unless it's on the vertical side. If you're new to stripping, you'll probably find that gaps that really need filling are almost non-existant. If you do have a really bad one. I use epoxy, sawdust and cabosil. The cabosil (or any white filler) helps keep it light color because sawdust alone will get very drk. If you don't have cabosil, use talcum powder, corn startch etc. along with the saw dust to get your color best you can.
But if you really want a non-visible fill. Cut a knife wedge of the same wood and tap it into the gap with some carpenters glue. I fill any end points or tiny notch openings with a wedge of the same wood. Just drive it in there with glue. Let it sit prowd. Once dry and sanded it will be totally invisable.