Gaelic-medium education in the primary school educates children in the full Scottish curriculum. Gaelic is the only language in the classroom for the first two years (sometimes know as the 'immersion phase') and increasing levels of English language teaching are introduced from at middle stages and by Primary Seven children are exposed to equal measures of both Gaelic and English and should be equally fluent in both languages, or 'bilingual'.
In most instances Gaelic-medium education is delivered alongside English-medium education. This means that few children are entirely 'immersed' in Gaelic as English tends to be the language of the play-ground, dining-room and administration areas. Glasgow and Inverness have dedicated Gaelic-only schools.
You should note that most Gaelic-medium schools require that children enter at primary one and two and will not accept enrolments later than that stage, unless children are already fluent in Gaelic.