Latest itunes versions make gapless aac files but it isn't possible to choose a true VBR (not costraint) encoding.
Using Quicktime PRO I can have true VBR files (first I encode to a mov file and then I "extract" the audio streaming to an mp4 file) but I cannot have gapless file.

Any suggestion?

If you're running OS X: X Lossless Decoder.

If you're on Windows: Use Nero's AAC encoder.