Hi Gizem and welcome to the forum,
I’ll come right out of the gate: Recommended for GABA-edited MEGA-PRESS is to start with a 27-ml volume. Even your larger voxel is just 12.5 ml (less than half), so you’d need to scan four times as long as the recommended 320 transients at TR = 2 sec just to get to the recommended SNR.
Yes, you’re getting a bit of signal back from going to TR = 3 sec, but you’re still way short of the recommendations. There is no replacement for SNR. You should increase your volume of interest or scan for longer. Being time-constrained by other planned acquisitions does not justify compromising on data quality (you’ll pay for short scans with decreased statistical power).
See also this thread for general Siemens protocol recommendations and these for MEGA-PRESS-specific ones on Siemens and Philips.
PS: Can’t access the Google drive, please change access rights.