Oof - I'm quite happy with recons but for some that would be a tough call... :p
Also you'd miss out on the first Dalek adventure. It's not the best maybe but it's probably the story that saved the show and gave it mainstream appeal.
I am a big fan of 60s historicals though and Marco Polo is where the first TARDIS crew feel like a unit.
I'm guessing the 2 uses you already know are 'joke' and 'to retch/nearly vomit'?
Another meaning of 'gag' is to physically tie something around their mouth to stop them from speaking (like in films when people get material or tape in their mouths to muffle their cries for help in a kidnap scene? That material would be the 'gag')
Obviously in this instance it's not a literal gag, although I'm sure there's plenty of people who wish it could be. A 'gagging order' is a metaphor for not being able to speak about things