I've never had luck other than friendship if they have Gem as their sun sign, rising sign, mars, venus, or moon. They/I get attracted due to my gem in 8th house.
Only in friendship but of course you are aware they are the sign of the twin and it is often said they have multiple personalities which go from one end of the spectrum to the other.
My husband (deceased) was a Cap and I'm a Gem. We were soul mates. Though we were very different we complemented each other very well. I loosened up his tie and helped him find his fun side, he helped me focus my energy and achieve my goals. He kept me accountable whenever I tried to make excuses for myself. I really appreciated my Cap's no BS approach to life. Also, Caps have a wonderful sense of humor that not everyone gets, or even expects, because of their serious reputation.
Another thing I like about Caps is that so far, they are the only other sign that I've found that does not get offended or upset when I need to get away for alone time. Perhaps, because the Cap needs their own alone time as well. Even my Aries boyfriend (who is similar to my Cap hubs in many ways) gets hurt if spend too much time away. Neither my Cap husband nor friends have ever been upset about this.
Now that I think about it, I've had better connections with Caps than Aqua or Libra, who I'm supposed to be the most compatible with.
If you sense that there might be some chemistry between your friends, I'd go ahead and set them up on a group date and see what happens.