It is unlockable, if I remember right it's down the Good route, you need the goodness maxed out, then at the first gate when you need to give Gomamon your essence don't do so inside, you must do so outside, do this a few times and eventually it will unlock a blowjob scene, and you will also need to pullout here, angering Gomamon.
Next you need to just wait for a little bit click on empty space to pass time, but don't click any of the option boxes this will trigger a new scene where you go to sleep, and Gomamon sneaks into your pants to get your essence himself since you won't give it to him properly and this will be the Bodyjob scene.