Okay, I tried it with the tags, however there is still that gap remaining. After all, I purposefully did NOT include a break in the song, because I didn't want any noticeable silence to be there. And those songs which have a break already loop nicely the way they are. What's more, the .ogg files are actually larger than the MP3s I created, so it doesn't even make the download easier.
I do wonder though how the original tracks included in NeoLemmix loop without me noticing? The only track which has a break in it is "Ten Green Bottles", the others seem to go on indefinitely.
That's also the way the loop in Logic works - I guess there's some kind of crossfade going on in the background, blending the ending of the first iteration with the beginning of the second one.