Where to stay before GAS tour

Just as a general info: Berlin has two main events (IFA fair and Marathon) in September which makes hotel room prices expensive.

Close to Koblenz you might consider Cologne with several day trips such as Aachen or Königswinter and Bonn. Hotel prices are high in Cologne end of August (Gamescom fair).

We came in thru Frankfurt a couple of days early before the tour and stayed in Cochem which is smaller than Koblenz but a lovely town on the Moselle with a castle and vineyards.

Frankfurt is a good place to stay and get over jetlag. There is plenty to see and do here. Lots of history and museums, markets, festivals perhaps.
You can then either take the slow way to get to Koblenz - regional train to Rüdesheim, then a ship to St. Goar, then finish by train to Koblenz, as this lets you see all the castles on the Rhein, or you can purchase ahead of time, an ICE ticket that will get you there in an hour.

Koblenz is a mid sized city, with a lot to see and do. It's a tourist destination, although mainly for Germans, Belgians, and Netherlanders. There's a huge fortress, a great military museum, a good city museum, the Eck, etc.

But Frankfurt is also good, Mainz is good, and you'll get recommendations for all the really small towns along the Rhine.

Berlin is fun, but do not try to get from there to Koblenz and catch a boat in the same day.

Hello
Since you ll see Koblenz with start of your GAS Tour I would choose another location before start

For ex Rüdesheim am Rhein

Small village with lots to see and do
and on the way from Frankfurt to Koblenz

Do boatrip on the Rhine as far as St Goar and back (full day)
Go with chairlift up to Niederwald Denkmal (Monument)
Visit famous Drosselgasse in Rüdesheim and have a glas of wine
and perhaps go to nearby Monastery Kloster Eberbach
Film Location "Name of the Rose" with Sean Connery

check bahn.de for train connection
There is a direction train connection from Frankfurt Central Station to Koblenz via Rüdesheim
Frankfurt-Rüdesheim 1h and on Rüdesheim-Koblenz 1h same train

..and a little Video about Rüdesheim

Reading the GAS tour description does stir excitement... "Starting in Koblenz, at the confluence of Germany's mighty Rhine River and the misty Mosel..." and the tour offers a day trip by bus up the Mosel River on Day 2 (to Burg Eltz and Beilstein.) I am more than a little surprised that it does NOT include a day cruise on the Rhine River (probably the main reason people visit here!) or a Mosel River Cruise either. Both are very impressive rivers peppered with castles, vineyards and great scenery that simply beg to be cruised. Also left out is one of Rick's favorite old-world half-timbered towns, Bacharach, along with similar towns like Braubach (which is only 10 minutes away from Koblenz!) Personally, I think everyone taking this tour from Koblenz ought to arrive early just for some time in Braubach:

Photo Gallery, 3 pages:
https://www.stadtbild-deutschland.org/forum/index.php?thread/7322-braubach-am-rhein-und-marksburg-galerie/

Marksburg Castle, a never-destroyed Rhine Castle from medieval times with tours in English:
https://www.marksburg.de/en/circuit/#/

Tips on Braubach:
https://delveintoeurope.com/braubach-germany/

Koblenz is a mostly post-war-built city, not the sort of "charming" old-world place you will find in the neighboring towns and villages. Besides Braubach and Bacharach, there are other charming Rhine towns that you can visit by train or by day cruise boat. Already mentioned: Rüdesheim. Not yet mentioned: Oberwesel, Boppard

It would be a shame, I think to just transfer yourselves between FRA airport and Koblenz without some time in these places. I would stay in one of these towns rather than Koblenz if I had extra time - these are popular towns that get well booked up in September, when the wine fests happen (maybe you have time for one of these too??) But Koblenz will work as an alternative if you can't find rooms in the others. Here's a map with transport options and towns there:

http://www.loreley-info.com/eng/rhein-rhine/walking-hiking.php

I would go at least a couple of days early to cruise the Rhine. You could stay in Koblenz if that makes it easier for you. I’ve stayed there on 2 trips. But I personally would choose one of the smaller towns ( I like Bacharach. ).

Things that I really like that are not on the tour (1). Cable car across the Rhine to Ehrenbreitstein fortress, (2) the ruins of Burg Rheinfels at St. Goar, (3) chairlift at Boppard. The view of the Rhine valley coming down is my favorite (4). Walking the wall and the Liebfraudenkirche church in Oberwesel and (5) for something really unusual, the short hike to Burg Ruine Stahlberg above the Bacharach ‘suburb’ of Steeg.