I always felt like an alien too, though I have been somewhat fortunate that for the most part I have been the only person who didn't notice I was smart (some complicated family circumstances meant school was never a focus and I never did terribly well). Once I found out dd was HG+ some kindly people suggested this should not come as a surprise. Apparently the blank stares I used to get during conversation were because people had no idea how to respond - not because they were dumbfounded by my stupidity. Nor were people deliberately making poor decisions, or trying to incite me with poorly thought out ideas - they simply couldn't foresee or understand the consequences of what they proposed. This was a revelation and has made much more tolerant. Unfortunately I didn't figure any of it out until my 30s, when I had dd.
Having dd didn't make me feel less if an alien, but it has helped me understand why I am one. There is a beautiful piece on the issues experienced by gifted adults. I think it's quite well known, so apologies if it has been posted here before:
http://www.sengifted.org/archives/articles/can-you-hear-the-flowers-sing-issues-for-gifted-adults
