He wouldn't want to, but Wei Ying might, and Lan Wangji would not want to stop him.

He wonders if Gui Bei is trying to sniff him out and lets him do so, come invade his space and get used to his smell to see if he's safe, trustworthy, or just to remember who he is.

As Gui Bei comes closer, he lifts his chin just a little bit, and pecks his cute twitching nose. But then goes back to letting him nuzzle into his nose and get a good feel for him.