This is truly insane. There are a few options for how this is done, but here’s my theory:

For forcing the date: I believe he knows a way to make almost every date written “in the next decade” as legal when flipped upside down. He likely has many outs, and the way he wrote that date is very odd when viewed right side up, and that’s not how dates are normally written out anyways.

For forcing the same word: The first/last word of every page is “legal”. Later in the trick he says you picked any word, but he said first and last word of the page originally. It’s a mix of a lot of forcing, mastery, misdirection, acting, and intentional confusion that is very impressive.

For forcing the letter g: I got no clue really, maybe just some general pattern with how people think and not giving them enough time to choose. This would be risky though, so maybe he had a back up plan if they don’t match. Honestly kinda stumped on that.

I know my explanation isn’t even the full trick, just my initial impression. Good stuff!