This has been discussed before but if one goes by the text of the novel, Olga is 19, Tatiana 20, Onegin 26, and Lensky 18 when we first see them. Four years later we have the ball where Tatiana is married to Prince Gremin: he shares memories of their youthful pranks and adventures with Onegin (who by then is 30). This makes Gremin maybe a bit older (nobody says exactly by how much), maybe something like 35-40. So perhaps a ten year gap or so between him and Tatiana.
Tchaikovsky on the other hand needed a lower voice for the operatic Gremin, so people usually assume he is much older.