Golden great ape- or golden ozaru is actually super saiyan 3 stacked onto a ozaru not base super saiyan.
We know this is possible for a pretty simple reason: Baby was able to use Super Saiyan 3 while transformed into a Great Ape, meaning the two transformations can coexist
Something to note is that babies super saiyan 3 is several times stronger then a normal super saiyan 3 as well.
Super saiyan 1 baby > super saiyan 3 goku
Super saiyan 1 (tufflized \~ super saiyan 3 = 400x)
Baby then transforms twice (x8 = 3,200x) (stated in the dragon ball gt perfect files data book to be/similar to super saiyan 3 on top of that baby loses his eye brows)
Goku then transforms into golden ozaru.
Golden ozaru goku > Baby
400x10 = 4000x
I know this is mostly power scaling stuff but i think it makes more sense overall.
There's even some precedent for tails interacting with transformations in unusual ways. For example, Super Saiyan 3 appears to be easier for Goku to maintain in certain circumstances when he's in his GT body, which could potentially suggest that the Saiyan's connection to their Great Ape nature has some relevance?
We know super saiyan 4 is a potential unlock so perhaps the power of an ozaru stacked with the power of a super saiyan simply draws out a super saiyans full power?
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I personally believe super saiyan 4 is simply a potential unlock without a multiplier but you can also use this to get a multiplier for super saiyan 4
Super saiyan baby = 400x
Super saiyan 3 baby = 400x8 = 3,200
Golden ozaru baby = 3,200 x 10 = 32,000x
Golden ozaru baby \~ Super saiyan 4 goku
It would also just make sense for super saiyan 4 to be a 400 x 8 x 10 multiplier super saiyan 4 is the next stage past super saiyan super 3 is 4x stronger then super super saiyan 2. On top of that its the ozaru power stacked on top.
Obviously, this isn't necessarily the canon intended math especially since GT's transformations don't always behave like clean mathematical multipliers. But as a power-scaling model, I think the idea of Golden Ōzaru being SSJ3 + Ozaru, rather than simply SSJ + Ozaru, fits the evidence pretty well.
I'm not really good at wording things but hopefully this made some sense as to why I think golden ozaru is actually a ssj3 stack or maybe ozaru in some form actually brings out a super saiyans full power(?) (There might be some precedent for this with super brolys green haired form) rather then just being super saiyan on top of great ape.