I like using epoxy putty, if the gap is in an area where sanding is not a good option. I use Milliput brand Superfine White grade. It has 2 sticks. Just cut off 2 equal amounts and knead them together. The great thing about this putty is that it can be worked with wet fingertips and tools, with just water, no noxious chemicals. It has a long working time, which is great for getting it to do exactly what you want. It needs about 24 hours to completely cure. The stuff is very versatile. It can be rolled into thin sheets with a rolling pin, just like dough.
I filled gaps on this vinyl model of a T-Rex last night:
It was used in the gaps left after gluing on the legs, tail, and lower jaw. I rolled it into thin little "strings" to press into the gaps. Some gaps were pretty wide, and needed skin texture added. For that I heated up the ends of a couple of pieces of sprue, then squished them onto nearby skin texture. That gave me a couple of texture "stamps", and they did a fine job of adding skin texture into the Milliput. The Triceratops on its side got sheets of putty added to fill in the texture on his...whatever that part is called. It was pressed in place with wet fingertips, then the excess was trimmed using a very light touch with an X-Acto knife.