My three kids spent Pre-K through some portion of elementary school in French immersion (this was a private school, not a program within the public schools, if that matters). My eldest was in the immersion school through 4th grade, my middle kid through 2nd grade and my youngest through 3rd grade.
Learning a second language through immersion was a good experience for them, or at least I can report that for the older two (jury is still out on the youngest since she is just going into 7th grade). My eldest took French for a couple of years in HS, then took the AP test. She also took German through AP. Middle kid took French in HS for three years, then took the AP. Middle kid took Spanish through AP. Both eldest and middle kid report that they felt the immersion experience made it easier to learn another language (German and Spanish).
Doing well on the AP foreign language exams not only got them credit (not the case at all colleges though), but it also means they had fulfilled the language proficiency requirement for just about any college. Fluency also landed my eldest an internship at a foreign embassy.
We did not think of them as gifted at the time they were in the immersion school, but classes in French definitely added some challenge to those elementary school years. My kids clearly experienced more boredom in the public school elementary program once they left the immersion school, though part of that might have been the pace of classes. The immersion school was not a gifted school and did not require testing (IQ or other) for admission, but there were a lot of gifted kids there. A good percentage of the classmates of my older two went on to attend Ivies or other elite colleges (immersion school only went through 8th grade).
I would vote for immersion as it sounds like there is not any more advanced/challenging work in the English only classroom.
