iTunes actually has a gapless tag, at least for MP4. I am not entirely sure how it works, though. I think it disables iTunes cross-fading capabilities for tracks marked as gapless.
In general I think there are two separate issues here. One is what a lot of software players mean when they advertise gapless support. If you have separate files and just start playing the next file after the first ended you will add an additional small gap. If you have a CD ripped that has continuous audio without silence in between you will hear those small interruptions.
So a player supporting gapless playback will ensure the audio of the next file to play will be loaded slightly ahead of time so that it can consecutively send data to the audio output. This is one way to understand gapless support. In this most basic definition you might still have pauses in between tracks if the silence is part of the end or start of the track. If you have ripped your CDs with the silence included and you want to have them played back as they are on the CD this now works well. If there is a bit of silence between the tracks on the CD you will also get that silence on playback, if not it will be without silence.
Now players with gapless support can also sometimes detect and omit silence at the start or end of tracks. In this case you would get continuous audio even when playing back files that have silence at the end. If this is actually wanted everyone has to decide for themselves.
Now the problem as presented here is probably the most sophisticated: It would allow you to either already rip the audio without silence or have the player remove the silence, but then opt-in per file and maybe depending on the playback (album playback or random shuffled playlist) to have the player add a small gap again. It’s for sure the most flexible, but I don’t know of a player supporting this.
The disc ID doesn’t tell you if there is silence in between or not. Also getting the length of an audio file can be (depending on format and encoding) surprisingly tricky and in some cases it is even just a good guess.