I could see Roger getting one shotted by Roger, but I don't think Garp and Big Mom fought at all.
Big Mom most likely fought against Saturn, during the Kuma flashback, both Big Mom and Saturn were not that far apart from each other.
Something must have happened that allowed Kuma and Iva to escape, so I do think there is a good chance that Big Mom clashed against Saturn.
Big Mom imo is top 5 in the Rocks Pirates but not top 3, I do believe she is at the very least stronger than Wang, Teenage Kaido, John, and the others.
But at the same time, I would say that she is weaker than Rocks, Whitebeard, Shiki, and Silver Axe.