Freedom of assembly, like the other 1st amendment freedoms, can be regulated by government so long, per prior SCOTUS decisions. They key is that restrictions must be content-neutral and can only pertain to "time, place, and manner," additionally surviving strict scrutiny (which, in plain English, means "there's no other way to do it"). This is bread-and-butter First Amendment court case stuff; any US government class that's at least halfway competent ought to have covered this.

The coronavirus bans on public gathering are fairly clearly permissible under SCOTUS precedent.