This is largely conjecture, but: I tend to think Broly maxed out at a level a bit above SSBKK Goku or whatever Vegeta. IIRC, Gogeta pretty decisively beat him, which I'd take to suggest that SSB Gogeta is somewhat above base Jiren. Base Jiren basically made SSB Goku irrelevant to about the same extent that Buuhan did to SS3 Goku, so that feels like Gogeta getting a relatively equal jump on the powerscale to what Vegeto had. Jiren's power up feels a bit nebulous, since he still ultimately can't really DO anything to Ultra Instinct Goku, but he does go from being trivially flattened to making Goku pretty much go all out and exhaust himself, so I think it's safe to say he'd probably wreck SSB Gogeta at that point. I'm less sure about what I think happens if Gogeta can do something like SSBKK, because it's something he should be able to do at least briefly, but hard to say how much stronger it would make him. I'm gonna say proooobably not enough to beat Jiren as quickly as he'd need to even if it puts him beyond Jiren. Ultimately, I think any amount of powered up Gogeta would have at least as much trouble with UI Goku as post-power-up Jiren did, which is bad for Gogeta, because he'll flame out before Jiren in any case.

On the other hand, fusions always seem to come up with some weird powers, which could come into play.