If your song is looping fine in FL, but then has the pause after you\'ve exported it, then I\'d recommend exporting to a format other than mp3. Mp3s are known for having that exact problem. You\'ll have better luck with .wav or .ogg.
I\'m sure you can find an ok audio editor that you can download for free if you do need to edit the length of your file. Maybe look around at hitsquad.com.
But, if you have a good loop in FL, then there really should not be any reason that FL can\'t export good looping .wav file.