Can you plug your guitar to your amp and then plug your toneport into the back of the amp (in the plug marked recording/phones on my mesa) then plug that to your computer so you are recording your amp tone?
or can't you plug it into your amp?
Hope that makes sense
Ok heres the deal.
Interfaces bypass your stock sound card giving better quality in the audio as well as less latency.
The toneports were made to direct connect your guitar, bass, or mic into them without anamp but you can bypass the modeling in Gearbox and go though your amp.

just hook a line from your record out and then run that to the toneport.
It would sound better to mic the amp if you can get a mic like the SM57 and XLR - TRS adapter.

Run your speaker cable for your PC speakers to the back of the GX. you may want a second set of speakers to monitor your PC for playback.
Go into the recording program of choice and set the input and output to the toneport.
some computer sound drivers will allow you to set the sound up so that you can play into the toneport and have the audio for monitor and playback go though your stock card so you dont have to deal with dual speakers.

Running the output of the toneport though your amp input isnt the best choice