Everything from spackle/polyfilla to superglue to epoxy putties to paint/acrylic medium has potential use in filling gaps; the size and character of the gap, as well as the demands on the resulting surface, determine which works best. Fine gaps can often be filled with CA glue (a thick/gel/gap-filling type or thin mixed with baking soda/sodium bicarbonate) or water-based acrylic fillers (LGS, Vallejo Plastic Putty, etc.).

The CA-based fillers are the easiest to deal with... if the gap is mostly filled during joining and dressed carefully. As CA cures, the hardness varies significantly - catch it at the right time and it sands like a dream... but too early makes a mess of everything and too late means it's so hard, you'll likely sand/file away more plastic than glue, resulting in deformation and obvious seams. Acrylic fillers (whether neat or with added filler material, like the marble dust in Vallejo's putty) can be smoothed or removed with water, which is handy, but are liable to shrink while curing, since the solvent is water which constitutes a significant portion of the total composition. The more filler, the less shrinkage... but also the less adhesion, 'workability,' etc. Expect either to have to apply multiple layers or overfill, then sand back.

Plastic sheet/foam wedges, spackle, and wood filler are usually reserved for the largest of gaps, mostly found in terrain construction or custom sculpts/extreme kitbashes that will have further surface cladding.
In a pinch, they'll do for large gaps, but require by far the most subsequent finishing work to look decent.

Epoxy putties are generally considered the 'happy middle ground.' They can, with some work, leave a fine finish after filling the finest hairline cracks. They can also, if your wallet and patience allow, fill an entire display case with a solid brick of polymer. Of course, their most common use is those 1/32"-1/8" gaps that happens during common assembly and conversion, or sculpting entirely new parts. There are two main types, one of which is more rubbery (GS/Kneadatite) and the other, which is more clay/ceramic-like (Epoxy Sculpt, Milliput, ProCreate). Both are strong enough for the job, both can be sculpted heavily, and neither shrink while 'drying,' but the working and cured properties are different enough to warrant discussion. The 'rubber' family (GS) is immune to water, instead using it as lubrication and requiring caustic solvents to thin. These tend to work more like chewing gum than clay and cure with a slight flexibility. The 'clay' family (Milliput, et al) can be thinned, i.e. softened and/or smoothed, with water and cure to a harder, more brittle finish. Great if you want to be able to drill/sand/carve the stuff, but trickier to sculpt into soft, organic shapes and more prone to chipping and separating. Luckily, the two types can generally be intermixed, leveraging the qualities of the two putties in relation to their proportion (more GS makes it stickier and more flexible, more Milliput makes it take better to abrasives, etc.).

The 'solvent' family is the last main category. This includes things like Squadron putty or the old 'spare parts/sprue in liquid solvent cement.' These are stinky, somewhat caustic, and slightly awkward to manipulate, usually, but bond with greater strength than any other type of filler and cure into a nearly seamless surface that behaves much like the surrounding kit parts. This stuff is nearly bulletproof (by comparison), can be used as both adhesive and filler material, and the result can be sanded, carved, drilled, scraped, scribed, etc. with no particular consideration.