The 250cc plus Gas Gas models went to 40mm Marzocchi forks in 2005, so unless you are talking about Tech forks from the last couple of years, which I think are 39mm, I don't think the GG forks will be any better than the sherco ones you have ?
As long as stanchions are ok just change the oil, and seals if they leak, this will make quite a difference to performance.
Ohlins are very good, see if you can find a used one, they are serviceable