An Etymological Dictionary of the German Language/gern
gern, adv., ‘gladly, willingly, fain,’ from the equiv. MidHG. gërne, OHG. gërno, adv., from the MidHG. and OHG. adj. gërn; to the latter correspond Goth. gairns in faihugairns, ‘avaricious’ (comp. Goth. gairnjan, ‘to desire, long for, demand’), OIc. gjarn, ‘eager,’ AS. georn, ‘zealous,’ Du. gaarne, OSax. gern. Akin to OHG. and MidHG. gër (without the partic. suffix n), ‘desiring, demanding,’ as well as to begehren, Gier. The Teut. root ger (from Aryan gher, ‘to demand violently,’ was confused with a derivative form in r from a root gĭ (ghĭ), allied in meaning; see Gier, Geier. Whether the Sans. root har-y, ‘to be fond of,’ or Gr. χαίρω, or Oscan heriest, ‘he will be willing,’ is connected with the Aryan root gher is uncertain.