I'd say both Bahamut and Tiamat are under-CRed at 25.
Anyhoo, to start Gru off, let's look at the HD and ability scores of his contemporaries.
Bahamut
53 HD
Str 53, Dex 10, Con 39, Int 35, Wis 36, Cha 35
Tiamat
49 HD
Str 49, Dex 10, Con 35, Int 28, Wis 25, Cha 28
Great Wyrm Iron Dragon
40 HD
Str 47, Dex 10, Con 31, Int 30, Wis 31, Cha 30
Great Wyrm Gold Dragon
41 HD
Str 47, Dex 10, Con 33, Int 32, Wis 33, Cha 32
Great Wyrm Red Dragon
40 HD
Str 45, Dex 10, Con 31, Int 26, Wis 27, Cha 26
So it looks like Bahamut is +6 Str, +6 Con, +3 mental stats over the toughest great wyrm of his grouping.
Tiamat is +4 Str, +4 Con, +2 Int, -2 Wis, +2 Cha over the toughest great wyrm of her grouping.
So for Gru, we could do something similar, perhaps +4 Str, +4 Con, +2 Int, +2 Wis, +2 Cha over the great wyrm iron dragon, giving him scores of Str 51, Dex 10, Con 35, Int 32, Wis 33, Cha 32.
I'd set his HD no lower than 46.
Thoughts?