It is possible that the Gemara did not mention Shimshon because there is no way that anyone would think to say that "Gibor" means the ability to do the acts done by Shimshon. However, Avner and Yoav were extremely well-trained warriors who honed their talents until they became exceptionally strong within normal human nature (ex., l'Havdil, a professional body-builder who is well versed in the art of fighting). Perhaps one would think that Gibor must mean such a person. The Gemara tells us that even this definition is too much, and that the general term "Gibor" refers to anyone who is physically intimidating.

All the best,

Yaakov Montrose