The Dragon Ball Super movie has a completely different art style compared to the anime. This is similar in comparison to what was said about the Super Saiyan Blue forms used by Goku and Vegeta. There was a lot of speculation that Goku and Vegeta were supposedly using a new version of Super Saiyan Blue, possibly Mastered Super Saiyan Blue against Broly. However, it was just the regular Super Saiyan Blue transformation with a new art style where the hair color seems to look darker.
The transformation Gogeta is using here is clearly Super Saiyan Blue. We see this based on the color of his hair. The different art styles and the superior animations in the movie compared to the series, makes it appear that way. Even Xenoverse 2 just put out a trailer with Super Saiyan Blue Gogeta.
Also Note: According to new arc of DBS in the manga, Goku tells Vegeta that he hasn't been able to use Ultra Instinct even once after the tournament of power, plus, MUI has silver hair and UI Omen has the special aura with regular hair and Gogeta clearly has Blue hair here. Secondly, the hair for the SSBE transformation is a lot darker. You can look at this photograph here, of SSJB and SSBE side by side.
While the manga had the concept of the "Mastered Super Saiyan Blue" being a unique transformation, the anime doesn't. However, I think it is more or less implied that Goku and Vegeta would have perfected their use of Super Saiyan Blue by now considering the fact that Goku can effortlessly stack Kaioken * 20 on top of it and has even ascended to levels far beyond SSJB. Vegeta too has broken past the limitations of SSJB to unlock a new form. It is more or less implied Goku, Vegeta would have mastered the Super Saiyan transformations by now. However, there is no indication or change in the transformation
Nevertheles, there is definitely evidence according to Spoilers that Gogeta fights Broly in Base, SSJ and Super Saiyan Blue. Hence, the form being used here is Super Saiyan Blue.
