What rhymes with gratitude?

Antonym is one.

Acronym, aluminum, anonym, antirrhinum, eponym, homonym, mediastinum, minim, molybdenum, paronym, per procurationem, platinum, polygonum, pseudonym, tautonym.

Near rhymes (spoken) consider "im-im" e.g. minimum, chrysanthemum, maximum, mesembryanthemum, optimum.