As with most things school related - it really depends.
I'm in Canada so French immersion (FI) is big up here (although the popularity varies from region to region).
I grew up out west in a city where French isn't very common. The people I grew up with that did FI there loved it. In general families with struggling students either pull their kids from FI or don't sign up in the first place so the classes tend to have a higher concentration of smart, studious kids. I don't know if the people I know would be considered gifted but I'd guess at least MG. I do know a couple of them had a bit of a challenge in first year Engineering since all of the physics/chem/math terms they knew were in French but many schools now keep the maths and sciences in English to avoid that. Overall the FI schools acted sort of like bright/gifted magnet schools in a city that didn't have any gifted programming at the time.
I now live in Ottawa where French is a bigger deal (higher up Government jobs require French and it is an asset for many other jobs). All of the public schools are FI at least for the first couple years and then eventually you can opt out (down to 25% French I believe). Once the kids are old enough to choose then you get the effect that the weaker students opt out but there are a lot more kids here that stick with it so it isn't the same as out west. One potential downside to FI is that for gifted kids that are the type that want to dive deep into topics the language limitations can be frustrating (or at least that is often mentioned on gifted boards up here). In our case my kids are both obsessed with math/science which are in English so I can't say if they would be less boring than if they were in French where at least the language would add a bit of challenge. My DS7 isn't a fan of French (he has a LD in processing speed so maybe that makes it harder for him to pick it up???). He is definitely the type to drill deep on subjects and I suspect having science in French would make him more frustrated but it is hard to say. My DD5 LOVES it. She is constantly singing in French and will pretend to speak it all of the time. Last night we played a whole game of Battleship in French just because she wanted to. It is really quite funny because she just makes up French sounding words to fill in whatever she doesn't know (she has a great accent so it sounds almost convincing). Another consideration is that it can be challenging to help with homework as they get older (assuming you don't speak Spanish). My kids are young but I have friends with older kids who have often called upon our Francophone friends for help. On the plus side, it makes it more likely that the kids are doing the work rather than over-involved parents
