A/N -- Sorry for not updating this EVER. I have a lot of ideas/theories, I just need to write them down here instead. Anyway, enjoy!
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In "PLAY FIVE NIGHTS AT FREDDY'S 2 IN GMOD!! - NEW Gmod Five Nights At Freddy's 2 Map (Garry's Mod)":
Ghost (at 10:50) mentions to Toast: "Did I tell you about the nightmare I had? Last time, where he wanted to go to space? But then he didn't? He went to a different dimension. That was also under the ground."
And how he "never got over that"?
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BACKGROUND INFO:
-UnderTale is a game set ^underground^ Mt. Ebott, a mountain
-Jordan (Venturian) did a neutral route throughout his playthrough of the game
-Jordan named his character "Jimmy," stating in the beginning of the adventure that the fans were "gonna have fun with this" (in reference to the naming of the character)
-A lot of people (including myself) theorize that the UnderTale playthrough was a part of Jimmy/Johnny's past (it couldn't have been Gregory anymore due to the naming of the character - Gregory was only called "Jimmy" after he went crazy because of Cardboard Friend)
Could Ghost have been referencing a distant memory (appearing in his dreams) of what could have been Casket's past in UnderTale, a possibly "different dimension"?