I haven't heard that there is even a buyout window available to start this season. So the obvious answer would be in June/July, whenever they do open up shop for compliance buyouts. In the meantime, Habs don't have any reason to buy him out.
About the only far-fetched scenario for an immediate buyout of Gomez would be if they had to clear tagging room vs. a $60M upper limit in order to sign Subban. But the unlikelihood of that, coupled with the lack of apparent buyout window, suggests an immediate buyout is not really something that could happen.
Gomez will be on the Habs roster for the remainder of the season, but, regardless if how he performs in the shortened year, he will be gone in the summer, well unless he bounces back with "100 points in the 48 games" (iiTzLaMia)