Capricorn builds a five year plan and checks it off in order. Gemini makes a plan on Tuesday and has three better ideas by Thursday. That is the modality gap right there: Capricorn is cardinal, wired to initiate and steer things toward a fixed outcome, while Gemini is mutable, built to adapt, sample, and change course whenever something more interesting wanders past. Left alone, Capricorn starts things and needs them finished. Gemini starts things too, from a different direction, and is genuinely fine leaving several unfinished at once. That difference shows up in small, real ways, like Capricorn wanting to book the vacation itinerary six months out while Gemini wants to decide the morning of, or Capricorn getting quietly annoyed when weekend plans shift twice before Saturday even arrives.
What saves this pairing is that Saturn and Mercury are not actually enemies, just different clocks. Saturn wants proof, structure, results you can point to. Mercury wants information, movement, something new to talk about. Gemini can talk Capricorn into loosening the schedule for one spontaneous night out, and Capricorn can talk Gemini into finishing the project instead of abandoning it for the next shiny thing. The earth and air combination, generally more friction than flow, actually works in Capricorn's favor here, because Gemini's ideas need somewhere to land, and Capricorn is happy to be that ground, as long as Gemini does not treat the relationship itself as another thing to sample and drift from.
The honest tension is patience versus stimulation. Capricorn measures love in consistency, Gemini in curiosity, and neither should have to fake the other's rhythm to be believed.