Edit: I was wrong. I don't know what I was thinking but the TV graphics show the gap from the leader, not between the groups as I thought. I could have sworn it was otherwise. I can only think that I have, for some reason, recalled it differently responding here despite seeing it time and time over again. Funny thing is, I can even hear the commentators voice in my head saying 'and a further x minutes back'
Both, depends on context, and maybe even presenter.
As a rule, ime, when watching on French tele, the gaps - as shown on the tv graphics - are generally expressed in terms of gaps between groups. But there are instances when a presenters might say x minutes/seconds behind the lead group, or such-and-such a jersey.
Edit: the reply above has got me wondering now! I always thought the gaps shown were between groups. I'll have to take note tomorrow!