Sharing a proof on ResearchGate? Inappropriate?
Take a look at your copyright agreement - many will differentiate between the preprint, postprint, and the final published version. 

Default language in copyright agreement tends to be most restrictive with the final published version and least restrictive with the preprint. 

You can always contact the publisher and specifically ask for permission. I've done this in the past and even gotten special permission to post the final published version (they asked I prominently add the publication and DOI to the 1st page).