Gas blowback rifles are a niche market because you can not make a reliable or practical GBB rifle without external gas. The upcoming Tanio Koba M4 and AKS74U will shake a bit during firing but you can not stuff enough propane into a mag to throw back a bolt of any significant weight. You will also get major cooldown problems which means unreliable consistency.
CO2 has enough pressure to throw a bolt back of significant weight, but powerlets are expensive and the CO2 needs to be regulated down from 800 psi. You cannot accomplish that in a small magazine without suffering from the same effects as propane.
I have a gas blowback M16 carbine that uses external gas like a paintball gun. Other models have been created as custom projects, a few are production models, and they are VERY expensive. Most people take one look and immediately dismiss them because of the external tank and hose. What they don't consider is the recoil is nearly on par with a 5.56mm without the muzzle rise, and the noise is FAR more gratifying than anything a whiny gearbox could ever hope to achieve. I didn't like the tank and hose either until I saw these videos, at which point I said "I don't care what I have to lug around; with that recoil and noise I'll live with it!" And I never, EVER looked back.
GBB M4 (customized; mine is similar):
GBB MP5 (production unit, I have one also):
Custom GBB FAL:
GBB M60 (production unit):
GBB MG42 (Shoei conversion):
GBB MP44 (Shoei conversion):
Never, EVER will you get that kind of performance from internal gas. If you could, it'd have been done already. Sad reality is you either live with the tank and hose or stick to AEGs, and to be honest the tank and hose are NOTHING to deal with at all. They fit nicely into a pocket or camelback and the tradeoff when you squeeze the trigger is definitely worth it.