one level, depending on the game.
In the PS1 version, Gritty Ditty is a hybrid ability learned by reaching rank 5 as a Warrior or Bard immediately after completing the other. When used, it increases the defence of all party members by one level for 0 MP, making it similar to Kabuff. In the 3DS version, as hybrid abilities no longer exist, Gritty Ditty is learned by Gladiators at rank 3, Luminaries at rank 5, and Druids at rank 1 and can only be used as those classes.
Gritty ditty can be sung by any character that has the 'Minstrel's Manual' (quest #102) item in their inventory. In this entry onwards, the song actually increases the attack of all party members, rather than defence. It is also used by Genie sanguinis and Moai minstrels in battle.
Although unusable by any of the main heroes, genie sanguinis can use Gritty Ditty in battle to augment their allies' attack much like Oomph. If a genie sanguini is summoned with a monster medal, it will perform Gritty Ditty on the party.