Hi there Ace, how's it going!
I just reinstalled GT3 on my PC for you to see why this is happening, and I can confirm it does seem to happen with any effect when you hit 'Process'.
I've been through the help files and the options and I can't see any way to alter this behaviour. It's probably set this way by default so that any stereo effects are correctly applied though.
There is a way round it, if you really want to end up with a processed mono file. In the
Edit View, first select the track and then from the menu bar click on
Edit > Bounce to Track/s/... Choose a (different) Track as the destination for the bounce, then in the
Format drop-down list make sure you select:
Mix Stereo Content to Mono 
In all honesty though, there is a far better way to achieve what you're doing and that's to insert the effect you want to use in the Track's FX Bin. The 'Process' option is really a hang over from a time when computers didn't have enough processing power and/or RAM to run effects in real time - permanently changing the recorded audio (also called 'destructive editing') is a one-way process once you've saved and closed your project.
When you mix down your project, the audio will have the effect applied to it, but the original audio is never altered (i.e. non-destructive editing).
Hope that helps!