I think we've all played this before, where one item is behind the green glass door, but a related item is not. Example: a pinniped is behind the green glass door, but a seal is not.

I'll start us out: Boba Fett is btggd, but a bounty hunter is not.

A critical miss is behind a green glass door, but a critical hit is not.

An arrow is btggd, but a bolt is not.

Ironically, a Crossbow is behind a green glass door, but a Longbow is not.

A battalion is btggd, but a legion is not

Full Plate is behind a green glass door, but Scale Mail is not.

This looks interesting, but I have no idea how it's played. Could you provide a description please?

Studded leather is btggd, but hide armor is not

GobO_Sapper is btggd, but GobO_Fire is not

I would, but part of the game is to figure what the catch is. Go back and analyze what's different. It's easier to see when it's written out.

Yeah its originally a riddle, but as a game I dunno if 'the secret' should be spoilered in the OP or make people try and figure it out.

I'll give you a hint: A Green Glass Door could be behind a green glass door, but a Black Iron Gate could not. However if the gate was made of Steel, then it could be.

A game in the Forgotten Realms could be behind a green glass door, but a game on Dark Sun could not.

Hell is btggd, but heaven is not

Summer is behind a green glass door, but Winter is not.

Fall is behind a green glass door, but Autumn is not.

Plotting and mapping are btggd, but graphing is not.

Running and Sleeping are behind a green glass door, but Stealthing and Hiding are not.

Puzzles and riddles are btggd, but not traps or mysteries.

A Hood is behind a green glass door, but not a Hat.

Boots are behind a green glass door, but Shoes are not.