The Collaborative International Dictionary of English v.0.48:

Gratis \Gra"tis\ (gr[a^]"t[i^]s or gr[=a]"t[i^]s), adv. [L.,
   contr. fr. gratiis out of favor or kindness, without
   recompense, for nothing, fr. gratia favor. See Grace.]
   For nothing; without fee or recompense; freely; gratuitously.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gratis
    adv 1: without payment; "I'll give you this gratis" [syn:
           gratis, for free, free of charge]
    adj 1: costing nothing; "complimentary tickets"; "free
           admission" [syn: complimentary, costless, free,
           gratis(p), gratuitous]