Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
grātanter, adv., v. grator fin.
grātor, ātus, 1, v. dep. n. and a. [gratus], to manifest joy, to wish one joy, to congratulate, to rejoice with, rejoice (mostly poet. or in post-Aug. prose; not in Cic.; class. gratulor, congratulor): nec tibi me in hac re gratari decet, Att. ap. Serv. Verg. A. 5, 40: inveni, germana, viam: gratare sorori, Verg. A. 4, 478; Ov. M. 6, 434: invicem inter se gratantes, Liv. 9, 43, 17: nescia, gratentur consolenturne parentem (by zeugma), Ov. M. 1, 578: Jovis templum gratantes ovantesque adire, Liv. 7, 13, 10: laudantes gratantesque, Tac. H. 2, 29: inter venerantes gratantesque, id. A. 2, 75: ad gratandum sese expedire, id. ib. 14, 8; Ov. F. 3, 418.
With acc.: gratatur reduces (= eos reduces esse), congratulates them on their return, Verg. A. 5, 40; cf.: (eum Tiberius) incolumem fore gratatur, Tac. A. 6, 21 fin.: totoque libens mihi pectore grator, Ov. M. 9, 244; cf.: quid tibi grataris? id. H. 11, 65.
Hence, grātanter, adv., with rejoicing, with joy (post-class.): senatus gratanter accepit, Capitol. Macr. 7: accipere, id. Max. 14; Amm. 17, 12; Inscr. Orell. 2588.