Machine Gun Kelly says it took him all of 15 seconds to realize Kate Beckinsale is a good match for his buddy, Pete Davidson ... and their 20-year age gap makes perfect sense to him.
MGK gave us his official seal of approval Tuesday night as he was leaving The Nice Guy in WeHo. He told us, "I love that s**t" ... as soon as we brought up his homie's new bae.
As it turns out ... MGK's a big fan of older women too.
As you know, PD and KB have been hitting the town together a lot lately ... including this past weekend when they were spotted smoochin' at a Rangers game in NYC, and also hitting up the 'SNL' cast party.
As for why Kelly thinks Kate's good for Pete, it's pretty simple ... he pins it to the "rad" factor. He also says he knows why, lately, Kate's had a thing for much younger guys -- even though it hasn't always ended well.
Yes, MGK's biased, but check out his take.