Ganar (Spanish for “to win”) comes from the old Germanic root waidanjan, meaning “to hunt”. From the same root, via French, we get the English… gain.
The g‑n pattern is clearly visible in both.
Interestingly, this is almost an example of the w- to g- pattern, like guerra and war. It has the original w- root in the original word but the modern words, in both Spanish and English, use the g- sound (since the English word came indirectly from Latin via French).