I have a textured finish on my existing drywall and i would like to know if I overlay this wall with another drywall and GG in between, will I still get the same sound proofing efficiency with a flat untextured wall?

First, I put a "million" drywall screws in the existing walls and ceiling to make it solid. My walls were not textured, but the GG should seal the two OK. Then I put another million screws thru both drywalls, which also helps the GG seal. The end result is a firm barrier. You can pound on it and it feels like a brick wall.

I did double drywall on all 4 walls and the ceiling.

I marked the floor where the studs were. Once the second wall goes up, it's hard to know where to mount the drywall screws so they go into the studs. Occasionally, one screw would hit the screw in the first drywall. But move over a fraction and screw again.
Take Aim

First, I put a "million" drywall screws in the existing walls and ceiling to make it solid. My walls were not textured, but the GG should seal the two OK. Then I put another million screws thru both drywalls, which also helps the GG seal. The end result is a firm barrier. You can pound on it and it feels like a brick wall.

I did double drywall on all 4 walls and the ceiling.

I marked the floor where the studs were. Once the second wall goes up, it's hard to know where to mount the drywall screws so they go into the studs. Occasionally, one screw would hit the screw in the first drywall. But move over a fraction and screw again.
Take Aim
i would probably end up with what you did. but my only concern is that my existing wall is heavily textured and i am thinking that even though the screws will squeeze the GG with the overlayed drywall it might still leave airpockets wherein sound might escape. another option i am thinking is to scrape off the texture from the existing wall before overlaying it with GG and a 5/8" drywall.

How textured would you say it is? Average texture or popcorn is OK. You might consider getting a pole sander and sanding screen and knocking down the high spots. You can rig up a vac attachment to keep tidy.

Otherwise, GG is successful on such textures.

QUOTE: "...it might still leave airpockets wherein sound might escape."

That's not how Green Glue or damping in general operates, so no worries.