Both Goten and Trunks showcase the same competitive personality the majority of kids in real life have, and are more or less the same type of rivals as Vegeta and Goku in DBS, close friends who love competing with each other; therefore, Gotenks would have a rival-boost, assuming it exists. Which goes hand in hand with the statements we had at hand regarding Gogeta's stature.
Captain Cadaver said:
I don't see why it wouldn't apply to Gogeta. After all, Elder Kaioshin shouldn't have been aware of Goku and Vegeta's rivalry to much of an extent beforehand and likely took that into account from his observation of their interactions just prior to fusing. Meanwhile, Goku wasn't aware of the rival boost, so he wouldn't have taken it into account when discussing Metamoran fusion.
Gotenks was (possibly) unknowingly amped by the rival boost and Goku's estimates on Gogeta take into account the power of Gotenks; herefore, even if it applied to Gogeta, the estimates would already be taking it into account.
Furthermore, Elder Kaioshin states that, because it was Goku and Vegeta, the
Potara was able to go so far.