Aaaaaaalright. I know this thread is not active since 2014, so I doubt that my answer would help the OP. But, since it is also the first result that Google showed me when searching for "UNP texture seam on legs", and in the first page of results I found no useful information, and I managed to fix it for myself, I guess I'll leave the answer to which my problem was for if someone finds it useful:
As Jeir states, if your textures are missmatched for your bodytype sure problems would arise. My problem was that I double-triple-eneadly checked that the textures were apropiate for the body. But the gaps still appeared. The fault was in my side of course.
As adventorous as I am, I was using GIMP to do some fine tuning to add some details I wanted to the body textures... The way I discovered there should be something in this process that was messing things was that reinstalling original UNP textures manually fixed the gaps (I was not using original UNP textures as a base in my project but Demoniac's, though that shouldn't matter). Then if I simply took the working main UNP texture, added it as a layer in my project, recombine all the layers in it and then export it again the gaps reappeared. So... despite beeing a total noob with textures, GIMP or anything related I considered that only format or size could be the variables changed...
And as such discovered that my exported project had a size file of 4096x4150 instead of 4096x4096. The offender: One layer that I had moved around outside of the image canvas but I didn't knew that images outside of the canvas already add their size to the total image size. So I croped the image size to the canvas size and everything worked again.
TL;DR: If you see gaps and your texture should be apropiate for your body type, check the size of the image. For a body it should be 1024x1024 or 2048x2048 or 4096x4096 or 8192x8192. If both numbers are not matched, that's probably the cause of the gaps.