It depends on how much gap and what the inlay is. For small gaps around brass/silver inlays, you can remove the inlay and peen it where the gap was and it will stretch the inlay, then reshape the edge with a file and re-inlet. If it's a lockplate, a minor gap can be closed the same way and a major gap (within reason) can be closed by running a small bead of weld along the edge of the plate to enlarge it, reshape and re-inlet. Barrel tangs can be encased with a brass inlay as part of the decor to cover a gap. A gap at the barrel breach can be closed by either moving the barrel back or soldering a silver plate to either side of the tang/lug and filing it down to the barrel, it will appear sort of like a barrel band. bear in mind that it will have to be removed to pull the breech plug so do it after the TH liner is installed. For serious gaps around barrel tangs and locks and such that require a wood "patch", rather than adding a sliver of wood, I would prefer to cut out the whole area and replace it and re-inlet. Make the patch a size and shape that you can incorporate a carving line right along the seam and it will completely disappear. However, if you have the ability to do this right, odds are that you didn't create gaps in the first place. This is more for cases where a serious flaw in the wood shows up.