There aren’t very many English words ending in <gain>: again, against, bargain and gain plus a handful or so derivatives and compounds.
Again and against are pronounced əˈɡen(st), or əˈɡeɪn(st). Bargain has these variants: ˈbɑːɡɪn, or ˈbɑːɡ(ə)n, and ɡain is quite simply ɡeɪn.
This is a guest tip by Petr Rösel. See his blog here.