I've got two kids in GME and I can report that their classes are absolutely rammed full as is the whole building, so we're not in it for small class sizes or swanky facilities. What they have access to is a richness of culture, an appreciation of music, a strong community spirit, dedicated staff who see the individual child and an ability to think flexibly that comes from bilingualism. Both my children, who are at opposite ends of the academic spectrum, are thriving in GME.
They don't always speak Gaelic in the playground, but they do speak a mix of Gaelic and English with their friends, at shinty, at dancing, in music lessons. They both go to a lot of extracurricular stuff that's conducted primarily in Gaelic.
I'm not a native speaker, but I've spent more than a decade learning in order to support my children's education. It's not an easy option by any means and takes commitment. It's a choice we've made as parents, so I'm very aware of our responsibility to support our children as much as possible.
It's not a choice that suits everyone, but I'm not sure why people would want to take that choice away from others.