It is unenforceable from the start. All the major world powers would continue their research (perhaps slightly less openly) simply from a MAD angle (it is irresponsible to ignore the value of a pathogen that no one else has seen and you have the antibiotics for).
We are living in dark times in terms of our technological capability and the aggressiveness of state actors.
I would argue that the only chance we have is to reign in the behaviours of our states. Crazy and seemingly impossible, but stopping science/tech is far beyond reach.
So, while it may not be 100% foolproof, it would be quite meaningful.
I don't think there's anything that we could learn from gain-of-function research on deadly viruses and bacteria that would be worth risking millions of deaths.
It doesn't matter how good your intentions are when your behavior is extremely dangerous and can (and possibly did) result in a global pandemic that kills millions of people. Risk/reward calculations should be performed without regard to intent.
>Banning it would reduce the amount done greatly.
It would also reduce the risk of a man-made virus killing millions of people.