Aries moves like someone who decides on Tuesday and books the flight by Wednesday. Gemini is happy to go, just don't ask them to commit to which airline until the morning of. That is the cardinal and mutable difference right there: Aries initiates, Gemini adapts, and together they actually make a surprisingly good travel team, because neither one needs the plan to stay fixed once it exists. Aries picks the direction. Gemini keeps it interesting once you're moving. Where it gets sticky is when Aries wants a decision locked in and Gemini is still turning the idea over out loud, weighing three other options just for fun, not because they doubt the first one. Aries can read that as flakiness or even rejection, when really it's just Gemini's mind doing what it does, restless and exploratory, with no urgency behind it.
The air and fire connection helps smooth this over more than people expect. Air feeds fire, so Gemini's chatter and idea-hopping actually keeps Aries energized rather than drained, and Aries brings a directness Gemini finds refreshing after all the overthinking types they've dated. Mars and Mercury get along fine too, action and communication working in tandem rather than against each other. Conversation between them tends to be quick, funny, a little competitive, never dull.
The honest friction is patience. Aries wants to argue it out and move on. Gemini wants to talk around it from six angles before landing anywhere, if they land at all. Neither is wrong, but Aries will need to let some things stay open-ended, and Gemini will need to actually close a loop now and then, just to prove they're still in it.