Goku didn't contribute energy to Gohan's Kamehameha, he just encouraged him to put everything he had into one moment when Cell wasn't paying attention.
"Seeing this chance and encouraged by Goku, Gohan unleashes all the rest of his power and power channeled to him by the spirit of Goku[3] into the Father-Son Kamehameha, overpowering the Solar Kamehameha and destroying Super Perfect Cell once and for all."
http://dragonball.wikia.com/wiki/Father-Son_Kamehameha
Why would they name the move the "Father-Son Kamehameha" if Goku had nothing to do with its power? Even Xenoverse states that the Father-Son Kamehameha combines both energies of Goku and either Gohan or Goten.
As for Buu, Vegito was toying with him the whole time so he could goad him into absorbing him and have the chance to cut everyone else out from him. How are you going to ask me if I even watched DBZ when you left out all of these important details? Especially considering that the Spirit Bomb was powered by Earth, Other World, and a hell of a lot more sources of energy than the one in the Frieza saga did.
Vegito's power level was way under Majin Buu's, and Goku's pride made it so that Vegito couldn't be formed again while facing Kid Buu. This is another critical point, if Goku was willing to accept help in the past like in the Cell Games then why was he so hellbent on destroying Buu on his own? This further cements the "Goku gets the spotlight" treatment, becoming Vegito again wouldn't have had the need to power the Spirit Bomb which I will give you credit for.
Right, but there's also inconsistencies in GT that prevent it from being canon. It counts the movie villains as part of the series (because when Hell breaks loose many villains from both the original series and movies are shown running amok), it completely throws the lore on its head (Vegeta gaining the Blutz waves and turning into a Golden Great Ape made no sense for him having no tail because it was explicitly stated before that the glands necessary to transform into a Great Ape are only present in the tail), and the character's feats themselves are so inconsistent. We have Goku withstanding Nuova Shenron whose as hot as the sun, then at the same time he expressed shock and surprise at Omega for blowing up a single city, gets cut by glass, and two SSJ4s struggle to hold up a single building.
Nitpicking here.
-The villains from the movies could have been killed offscreen by someone else. Just because they are from the movies doesn't mean Goku fought them in that timeline. Not one of the villains from the movies addresses the characters like they have fought each other before.
-Vegeta going Super Saiyan coincidentally after Goku can be used as an argument as well. Vegeta had darkness in his heart still by the time he went Super Saiyan and still couldn't quite control his ki, how is it that right after Goku attained SS that he did it too? Nowhere in the Japanese sub does it state that Saiyans need to have the tail go Super Saiyan, they can if they want to get to Great Ape form but not Golden Great Ape (in the Dragon Books, it says that once someone goes Great Ape any form of Super Saiyan is theoretically accessible) . And even if it did, Vegeta gained SSJ4 artificially, not naturally.
-That doesn't make GT not canon, as it has nothing to do with time placement or misleading events that deviate prior history. It's just mediocre writing, something that the other 2 series had.