> As far as I know we simply do not have evidence if or if not gain of function research played a role.

You can examine phylogenetic distance -- the degree of evolutionary drift that the initial virus had from others which are subject to natural (in-the-wild) evolution. COVID is way out there. And even then the closest "natural" virus had its genome published by the Wuhan lab long after the initial outbreak.

The lack of any close relatives suggests that gain-of-function very well may have taken place, as does the heightened vulnerability of typical lab-animals. What there isn't much evidence of is zoonotic transfer.