Source of gaps, from the most common to the least common :

Nero inserts gaps. Solution : remove them everytime you burn a CD from audio files.

MP3 have gaps. Solutions : crossfade, auto detect, test on silence with adjustable threshold, etc... Exception : Lame Mp3s specify how much gap they have and Foobar2000 removes them by default.

CD introduce gaps between files that are not multiple of 588 samples. Solution : burn with Feurio with the gapless option, or with a mastering software like SoundForge, CoolEdit, Wavelab, Samplitude...

The files were ripped with a remove gap or kill zero samples option, which ruins the transitions between tracks. Solution : re-rip the original CD properly, with all its original gaps and samples.

You listen to the CD with a software, non gapless player, like Windows CD Player. Solution : get a gapless player, like Foobar2000, or Winamp 5, or use a real CD Player.

You burn in TAO. Solution : burn either in SAO or DAO.

You can't burn in SAO or DAO. Solution : upgrade your burning software so that it recognizes properly your burner.

Your burner can only burn in TAO. Solution : Time to replace your 8 years old 2x CDR burner with a 52x new one.

You get clicks between some tracks in gapless playback even when you encode gaplessly from gapless wavs. Solution : try Vorbis for encoding, less sensitive to this rare artifact, or use a declicker for vinyl records after having decoded the compresed files to wav and joined them, before burning them.