They are in fact related by marriage. Gratiano is Brabantio's brother. Brabantio is Othello's father-in-law.
Elizabethans also referred to elderly men as "Uncle," and we know that Othello is nearly old enough to be Desdemona's father. Therefore, we can assume that Gratiano is an older man, but Othello is probably referring to the in-law relationship rather than Gratiano's age.