No American English four-letter word ends with GAN, but here are some other words:
5-letter words
began, bogan, hogan, lagan, ligan, logan, organ, pagan, vegan, wigan
6-letter words
brogan, kurgan, longan, lungan, origan, slogan, wangan
7-letter words
wanigan, yatagan
8-letter words
cardigan, hooligan, larrigan, misbegan, mulligan, nonpagan, toboggan, wannigan
9-letter words
brannigan, ptarmigan, suffragan
10-letter words
balbriggan, interorgan, shenanigan
11-letter words
centimorgan