First off, welcome to the world of enduro. Trials riders are a pretty tight bunch, I'm sure you'll find that the Gas Gas enduro riders are equally friendly.
I think I've made my feelings on the 200 perfectly clear in other posts. It's a perfect compromise between 250 power and 125 control. In my opinion, it has "just enough" power to get you through the tough stuff but not too much to get you into trouble. Of course if you are over 6' or 200 lbs, the 250 or 300 are probably better bikes for you.
If you've been riding trials, theres a rule of thumb with trials bikes... buy the bike you're going to ride right off the bat, regardless of age or skill level as long as the bike fits you. You get used to the power delivery and feeling the bike has. I've seen 12 year old trials riders who ride 280's and 40 year old riders who ride 200's.
We'll assume you're under 6' and 200 lbs, pick the bike to suit your riding style. Tight and technical, go for the 200. Open and flat, go for the 250 and if you ride a mix of both... go for the 300. I wouldn't worry about a 125 unless you want to race a certain class that is restricted by engine size.
Regardless of which you choose, you'll love any of the sizes Gas Gas has to offer. Keep in mind, all the frames except the 125 (smaller) and 450 (narrower) are identical. The only real difference between the 200, 250 and 300 is engine cc's.
im dan and i'v got a gassa 250 trials bike.Im in the market for a 125 enduro bike as im just about to turn 17. iv been riding trials for a while and decided to move over to enduro. i know i want a gassa but i dont know wether to go for a 125 or a 250 anbody got any ideas 
