You are attempting to equate assembly for the purpose of fighting racism or exercising vitally important activities like voting, with "assembly" for the purpose of "sick gainz bruh". This is such a false equivalency that it's laughable, and honestly also a little insulting.
Free speech means freedom to shitpost, not just write literature.
Correspondingly, freedom of assembly is for stupid and pointless things like church, moderately important things like gym, and not just critically important things like insurrection.
The proliferation of “illegal” gyms suggests that many people feel similarly.
Yes it absolutely is. I have no idea why you think otherwise. The right to "free expression" is conditional as well. We prohibit certain "peaceful assembly" and "free expression" every single day and have done so for centuries. The classical example: you do not have the "right to free expression" to go into a crowded place and falsely yell "fire", because that would be a public danger. Similarly, going to a gym during a pandemic causes a public danger. "Right to peaceful assembly" doesn't apply.
"Peaceful assembly to work out" is in no way similar to "peaceful assembly for protesting". It's ridiculous of you to claim otherwise. It's so ridiculous that I can only believe you're trolling or trying to make an argument in bad faith.