The "Amp in" and "Amp Out" on the Traynor is equivalent to an effects loop, having one output of the GT-100 going to the Amp Out will do nothing on the amp, and on the GT-100, will cut out half of your stereo/panned sounds.
Try the headphones first, but note that the GT-100 has "Output Select" settings to shape the tone to the application used.
Output select set to 'Line Phones" for headphones or to a flat response system (i.e. stereo or PA system)
This applies speaker cabinet simulation to the tone.
"Combo" or "Stack" for plugging into the front input of the amp (using the amp's preamp to colour the tone). Keep the GT-100 preamps switched 'off' - effects only.
"Combo Return" or "Stack Return" for using only the power amp section and a guitar speaker/cabinet.
You will be wanting to use the GT-100 preamps but switch 'off' the speaker cab part.
Plug into the Traynor "amp in" with just the GT-100 L/Mono output.
Another setup option to try further down the track is Four cable method (4CM), allowing you to switch between the GT-100 preamps and the Traynor one (using the GT-100 S/R Loop).