http://www.rockbox.org/wiki/GaplessHowTo
The simplest method is to use a format/codec other than MP3. For instance, Ogg Vorbis, Musepack and FLAC are all supported by Rockbox (except on the SH1 Archos devices) and are capable of gapless playback natively, without any special encoding techniques or workarounds. Simply encode the tracks per the respective encoder's documentation and they should play back gapless with Rockbox. If your format of choice remains MP3, then you can still have gapless playback by using any recent version of the LAME encoder, which will encode gapless by default. From LAME v3.90.3 upwards Rockbox will decode gapless, files encoded by older versions or other encoders are not guaranteed to playback gapless in Rockbox. The current recommended version of LAME is v3.97