1) R' Moishe, I looked but could not find this in the Rabeinu Gershom. Perhaps you might be referring to what Rabeinu Gershom writes at the beginning of 49b (in some editions): "v'Eizeh she'Yirtzeh Yachol l'Hakdim" -- "He may offer first whichever one he wants." Rabeinu Gershom is referring to a scenario where he has enough animals for both Temidin and Musafim, as he writes at the end of 49a.

I find it rather surprising that Rabeinu Gershom writes that he can bring first whichever one he wants, because we know that the Tamid should be the last Korban of the day. In fact, the Gemara itself asks this question and answers that it is only a "Mitzvah b'Alma" that the Tamid should be the last. So when Rabeinu Gershom writes that he can bring first whichever one he wants, he does not mean that l'Chatchilah he can bring the Tamid first but rather that if he did, it does not make anything invalid.

2) However, this is rather different from what the Sefas Emes writes: "v'Yesh Lomar b'Dochak Al Tamid Shel Bein ha'Arbayim Iy Hikriv Kodem Musaf Kasher" -- "One can give a forced answer that it refers to the evening Tamid that if it was offered before the Musaf this is valid." The Sefas Emes writes it in a b'Di'eved fashion, unlike Rabeinu Gershom.

3) The Keren Orah writes: "v'Ikar ha'Revusa Hu d'Musaf Kasher Gam Achar ha'Tamid" -- "The main Chidush is that the Musaf is valid when offered after the Tamid." Again, like the Sefas Emes, the Keren Orah is speaking b'Di'eved, and expresses it differently from Rabeinu Gershom.

I think that if we would have cited this idea in the name of Rabeinu Gershom, it would have made things more complicated, since we would have had to stress that he is not really talking l'Chatchilah, unlike what one might have thought at first sight.

Dovid Bloom