An Etymological Dictionary of the German Language/Annotated/Gast
Gast, masculine, ‘guest, visitor; wight; sailor,’ from Middle High German and Old High German gast (plural gęste, gęsti), masculine, ‘stranger, guest’; common, in the same sense, to Teutonic; compare Gothic gasts (plural gasteis), masculine (compare gastigôds, ‘hospitable’), Old Icelandic gestr, ‘guest (uninvited),’ Anglo-Saxon gyst, giest, masculine, English guest, Dutch and Old Saxon gast. Teutonic gastiz, masculine, ‘stranger, unbidden or chance guest from some foreign part,’ from pre-Teutonic ghostis, which left derivatives in Latin and Slavonic; Latin hostis, ‘enemy,’ properly ‘foreigner, stranger,’ Old Slovenian qostĭ, masculine, ‘guest'; with Latin hostis, ‘foreigner,’ hospes (properly *hosti-potis, ‘host’?), might also be connected. It is more than questionable whether West Aryan ghosti-s, ‘stranger,’ is properly ‘eater, devourer,’ and belongs to the Sanscrit root ghas, ‘to eat.’ It is worthy of notice in how many ways Teutons and Romans have transformed the idea underlying the old inherited word for ‘stranger'; the Roman regards him as an enemy, among the Teutons he enjoys the greatest privileges — a fine confirmation of Tacitus' account in the Germania. This evolution of meaning would be still more remarkable if the view were correct that Latin hostis, ‘stranger,’ is related to Latin hostia, ‘victim’ (stranger = ‘one to be sacrificed’?); this collocation is alluring, but very uncertain.