The "Gapless Playback" checkbox that used to be in iTunes was always, in fact, a misnomer.
iTunes since v7 has had support for gapless playback; self-determined & not adjustable by the user. The option for gapless playback in an album's Get Info was, in fact, to tell iTunes not to apply cross-fade to any album with that selected.
This would, of course, only apply if the album was played in strict album order, not in random playlists or shuffles.
One other 'feature' of gapless playback is… iTunes will only determine it for tracks downloaded from the iTunes Store, or from imported CDs, but not for random imported mp3s etc.
This leaves you with only one option -
Ensure cross-fade is switched off, which will prevent the early cross to the next track.
It will not, however, re-instate true gapless playback on files that did not come from iTunes.
Edit
There is the possibility to edit in the information manually [usual caveats to work on a copy would apply]
This is untested as yet.
In the iTunes xml file there is a key in every gapless track…
<key>Part Of Gapless Album</key><true/> which appears to always follow the sample rate line
<key>Sample Rate</key><integer>44100</integer>
You could try adding this line to all tracks in your album & see if it works without having to break the .itl file & make iTunes rebuild it.