Any recommendations about where to fly into before the GAS tour? Frankfurt or Luxembourg? If you chose Frankfort how difficult was it to get to Trier? How are the connections from Paris?
I flew into London and spent a couple of days there. It's a short flight from London to Luxembourg. There's a bus that gets you to the train station. It's a short ride to Trier.
Another option is to spend some time in Paris and hop the train from there.
Where are you flying from? Frankfurt has many options for direct flights from North America. When we did GAS we flew into Frankfurt. Taking the train was easy since there is a train station right in the airport. We took the train to Bacharach on the Rhine (its on the way to Trier) and stayed there one night, before heading for Trier and joining up with the tour. There are other places equally nice between Frankfurt and Trier worth staying at.
I flew in to Paris and spent 5 nights there before traveling to Trier last summer for the GAS tour. A Trip Report from 2015 put me on to this and well...I can't pass up a chance to visit Paris, lol!!
I took the TGV from Gare de l'Est to Saarbrucken, then changed at Saarbrucken for the regional train to Trier. I left Paris at 9:06, got to Saarbrucken at 10:55, left Saarbrucken at 11:19 and got in to Trier at 12:28.
I purchased my tickets on CapitanTrain which is now Trainline.eu. You can do that or purchase via the German train site DBahn. I don't think you can do the whole trip on the French Train site SNCF as you've got one whole segment of the trip that is solely in Germany.
BTW, there were several choices for routes including thru Luxembourg.