In theory, yes. In practice, no.
First the gas "lost" isn't really lost completely. So even though there's a small pressure drop, it is very VERY small, percentage-wise.
Second, most gas actions tap the gas fairly near the muzzle, so there's little time left before the bullet clears the muzzle - and nearly all the bullet speed already attained.
Third, you'd have a hard time deciding, because the individual variation from shot to shot is almost always larger than the loss of speed due to gas bleedoff.
All these can be clearly demonstrated by simply blocking or turning off the gas system (in guns like the Garand where you can do so.) Measure bullet speed with the gas system on and off and compare.
