gain has 27 distinct synonyms spread across 12 of its 13 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its 3 recorded opposites are fall back, reduce, loss.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
increase (one's body weight)
“She gained 20 pounds when she stopped exercising”
a quantity that is added
“there was an addition to property taxes this year”
the advantageous quality of being beneficial
the amount of increase in signal power or voltage or current expressed as the ratio of output to input
the amount by which the revenue of a business exceeds its cost of operating
Synonyms, antonyms and the broader and narrower relations for gain come from Princeton University’s WordNet. Full sources and licences.