why is it so important for your wife to swallow? at least she tried it and she was honest with you. if its realllly that important then tell her its something you really enjoy but dont push it again if she says no and isn't comfortable with it.
alternatively, instead of having your cock in her mouth while you're cumming, she could stroke you off until you cum and just have it shoot in her mouth....maybe that would help her gag less? but get your aim right
try eating better foods, lots of fruits and stuff. that could help your cum taste better