I think he could potentially beat anyone at lightweight. However, at welterweight he battered 5'10 formidable welter Mike "Twin" Sullivan, dominated the crafty Jack Blackburn who was 5'11 and 10-15 pounds bigger than Gans on some occasions, and outbrawled the great Barbados Walcott in the first two rounds of their 1904 draw. Walcott's arm got injured in round 3 but he still made it to the end Who's the best fighter you see him scalping at WW?
Gans in Goldfield may be the very best ever preserved on film, and he was weight drained for that one, something which wouldn't have been the case at 147. He drew over 20 with WW Champion Barbados Joe Walcott. Sam Langford went the Championship Distance with Gans and considered the Old Master the GOAT. Frank Erne TKO 12 Gans may be the greatest successful title defense stoppage win ever. Who could he NOT have beaten at 147? SRR, but Robby would have to take the Old Master the Championship Distance to do it. Man, I look at all the other WW Champions through the end of the Championship Distance era, and aside from Robinson, I just don't see anybody Gans definitely could not have beaten.
As WW Champion, we have footage of Walker's LHW challenge of McTigue, where the Toy Bulldog moniker becomes evident, as Mike forces him back, then Mickey blasts away as the 170 pounder moves in. After Robby, Walker might be number two at WW.