gain-

1. Prefix meaning "against", "contrary to", "in opposition to", "counter-".
2. Prefix denoting reciprocal action; "in return"; "counter-".
3. Prefix denoting restoration or a return to a previous state; "back again".
4. Prefix denoting repetition; "over again"; "anew"; again-.

Etymology

Inherited from Middle English: gain-

Inherited from Old English: geġn-

Cognate with Saterland Frisian: juun-

Cognate with Western Frisian: tsjin-

Cognate with Dutch: tegen-

Cognate with German: gegen-

Cognate with Icelandic: gagn-

In summary

From Middle English gain- (prefix), from Old English geġn-, gēan- (“back, against, in return”, prefix), from Proto-West Germanic *gagin, from Proto-Germanic *gagin (“towards, against”).
Cognate with Saterland Frisian juun- (“counter-”), West Frisian tsjin- (“counter-”), Dutch tegen- (“counter-”), German gegen- (“against, toward, at”), Icelandic gagn- (“through”). More at again.