There are two issues here.

Firstly, an encoder will insert blank gaps at the start and end of a track, which can last anything from 1/32nd of a second to maybe a quarter of a second in some cases. You can trim these with an MP3 editor utility - tfabris is a bit of an expert at this, and will no doubt provide a link to some suitable software.

The second issue is with the player itself. Even properly trimmed MP3 files can have a tiny gap at playback, due to a number of issues within the software. This shouldn't exceed 1/32nd of a second, but it is audible - just. I listen to a lot of mixes and I don't find it so severe as to be a real bother.

Rob