It depends on the person’s intent and the situation. If someone is trying to get to their essential work job and they are forced to ride on a crowded bus because there’s no other way for them to get there, then it’s not a sin, as you have to get to work. If on the other hand you decide to break the rules to have a big party or play basketball with a bunch of your friends when we’re all supposed to be social distancing, then it could be a sin; there’s no need for you to be playing ball or having a party. Whether it’s grave sin or not depends on many different factors.
Like most sins, it’s a matter of common sense, not bright-line rules. I also don’t think it’s particularly helpful to discuss these questions with people other than your priest, because you’ll get a variety of opinions.