Even if these researchers have good intentions, they're human, and humans make mistakes. If someone doing gain-of-function research lets a new deadly virus or bacteria out into the wild by mistake, their mistake can can cause a pandemic like the one we're currently suffering from.
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.
To be clear, I would guess that if SARS-CoV-2 was created in a lab in Wuhan, it was created with good intentions, and escaped from the lab by mistake. And rather than figuring out where to point fingers, we can work together to prevent future pandemics by preventing research that creates deadly diseases.
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.