Commentaries from dragon ball characteres many times tell us which character is stronger than other. But sometimes they seem to be wrong or contradict themselves later. With regards to Beerus, in the Dragon Ball Super manga in the times of the battle with Zamasu, it was said by Wiss if I recall correctly, than Gogeta might be as strong as Beerus. A lot of battles and training happened since that, several zenkais and Goku and Vegeta got stronger. Now in the Dragon Ball Super Broly movie, it was said by Goku that Broly might be even stronger than Beerus. If Broly is equally or stronger than Beerus, Gogeta has to be way stronger than him, since Gogeta showed to be much stronger than Broly. Is this so? Were Wiss and Goku just speculating or are there more clues sustaining the authors intended to assert this?
No it hasn't been proven Gogeta is stronger than Beerus. The only characters who have been proven to be stronger than Beerus are MUI Goku and Limit Breaker Jiren.
- Firstly, I believe the only character in the series who has the closes idea of how strong Beerus really is would be Whis. Hence, I would mainly be considering statements made by Whis with reference to Beerus's True Power.
- Whis had said several times that Goku + Vegeta working together would allow them to go toe to toe with Beerus. By Working together, Whis implies fusion. This could mean, Gogeta or Vegito could be potentially strong enough to defeat Beerus or be very relative to him in terms of power at the very least.
- Another important factor to consider is the time limit for fusion. Considering how powerful Beerus is, the fusion might perhaps not last long enough for Gogeta / Vegito to defeat Beerus.
- Whis makes another statement in the anime saying that Mastering Ultra Instinct would result in Goku/ Vegeta surpassing Beerus. Whis also says this to Vegeta when he fights Beerus in the manga and loses the fight. This more or less establishes the fact that MUI Goku would be stronger than Beerus.
Now with regard to statements made by a few other characters,
- During the battle with Zamasu, it wasn't Whis but Shin who says that Vegito might be stronger than Beerus. However, I believe the show has demonstrated enough reasons why we shouldn't take Shin's word seriously. For starters, Shin thought SSJG Goku was strong enough to defeat Beerus, he thought SSJB Vegeta was defeating Jiren, Vegeta would struggle against Pui Pui, had no clue of what Ultra Instinct was. The character has been displayed as someone who is very inexperienced and often jumps into conclusions and is in awe of what happens afterwards or is usually corrected or mocked by the Elder Kai about the same.
- The second comment was made by Goku comparing Broly to Beerus and yes, if that were the case, Beerus would be significantly weaker than a Gogeta or Vegito. While Goku has a much more better notion of how strong an opponent is in comparison to Shin, I believe even he might not know the true extent of Beerus's power. For example, Goku was confident he'd be able to defeat Beerus after attaining the SSJG transformation the first time around, that he was using only 80% of his power(Despite Beerus being significantly more powerful in comparison to that). Even in the case of Jiren, the more and more Jiren began to reveal his true power, Goku was in shock. For example, in Episode 123, when Jiren revealed a fraction of his true power, Goku was in such extreme shock that he dropped from his Super Saiyan Blue form to his base form. Even in case of Frieza, Goku had no idea Frieza was hiding a level of power that high until Frieza revealed his Golden Form to Goku
So in the end, it comes down to whether one thinks Gogeta Blue is as strong as MUI Goku and Limit Breaker Jiren. There is an obvious disagreement between a lot of the fanbase with regard to this and there aren't any statements made in the movie or in Dragon Ball Super to help establish this. However, based on Dragon Ball Heroes at least, MUI Goku was indicated to be more powerful and it being the highest level of power of the MC, it does seem to make sense to me. However, that is up to you to decide.