Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
grātes (usually only in the nom. and acc. plur.; in the abl. gratibus, Tac. A. 12, 37), f. [gratus], thanks rendered (esp. to the gods: gratias agere being the usual form to human beings), thanksgiving: ut Ephesiae Dianae lauta laudes Gratesque agam, Plaut. Mil. 2, 5, 2: diis laudes gratesque agunt, Liv. 7, 36, 7; 26, 48, 3; cf. in the following the passages, Tac. A. 1, 69 and 12, 37: vobis (dis) grates ago atque habeo, Plaut. Pers. 5, 1, 4: grates deis immortalibus agere habereque, Liv. 23, 11, 12 Weissenb. ad loc.; cf. 23, 12, 7: dis populoque Romano grates agunt, id. 10, 25, 5; 45, 39, 12: grates tibi ago, summe Sol, vobisque, reliqui caelites, * Cic. Rep. 6, 9: grates dis agere, Liv. 5, 23, 3; 30, 17, 6; Tac. A. 13, 41; cf. also: nec esse, qui diis grates agendas censeant, Liv. 30, 21, 9: salsipotenti et multipotenti Jovis fratri … laudes ago et grates gratiasque habeo, Plaut. Trin. 4, 1, 2: laudes et grates reversis legionibus habens, Tac. A. 1, 69; cf.: Agrippinam iisdem quibus principem laudibus gratibusque venerati sunt, id. ib. 12, 37: vobis, o fidissime civium atque amicorum, grates ago habeoque, Curt. 9, 6, 17: dexteram ejus amplexi grates habebant velut praesenti deo, Curt. 3, 16, 17; Tac. A. 1, 69 Draeger ad loc.: Tiberius egit grates benevolentiae patrum, id. ib. 6, 2: o decus Italiae, virgo, quas dicere grates Quasve referre parem? Verg. A. 11, 508: grates persolvere dignas Non opis est nostrae, Dido, id. ib. 1, 600: grates rependere, Stat. S. 3, 3, 155: quibus pro bene factis fateor deberi tibi Et libertatem et multas grates gratias (cf. above: grates gratiasque), Plaut. Poen. 1, 1, 6.
Sarcastically: at tibi pro scelere, exclamat, pro talibus ausis, Di .. Persolvant grates dignas et praemia reddant Debita! Verg. A. 2, 537.