In Return of the King Pippin says he’s known of Gandalf for years but didn’t actually know him pre-quest and I like to imagine the same applying in reverse. 

Gandalf never met Pippin pre-quest but he knew who Pippin was bcos his reputation preceded him. Every time Gandalf passed through the Shire ppl would be like ‘you’ll never guess what the Thain’s son’s gone and done this week’ and he was like ‘fascinating tell me more’

& then when Pippin showed up in Rivendell w Frodo he was like ‘YOU!!!’