You keep the reputation earned on questions and answers that were migrated to other sites. This works like normal question migration: if you don't have an account on the site it was migrated to, you assume ownership (and reputation) at the time of account creation. Votes on migrated posts are migrated as well, and they are cast by the Community user until they're "owned".
As for other ways to recognize of the time and energy spent on Gadgets, we're open to suggestions. One idea is a bronze Area 51 badge for users who had over a few hundred rep on Gadgets.