Naturally clip automation creates one envelope per clip. If you only want one envelope on the track, you need use track automation. Unfortunately you can't automate track gain. The alternative is to put an ampty FX chain at the front of the ProChannel to act as a gain stage going into the FX (assuming that's why you're using Gain instead of Volume), or bounce the audio clips together so you only need one clip envelope; just be sure to remove the existing envelopes first to prevent their being rendered into the bounce.