I will have a free day, in Glasgow, and would like to take a coach tour, up to Inverary Castle. Is there a tour company, in Glasgow that offers this service?
Rabbies and Timberbush have tours to Oban which go through Inveraray.
You would have to check if they allow time to visit The Castle.
Various operators do day tours that include Inveraray:
Check also Rabbie's, Discover Scotland, Highland Explorer...there may be others.
Citylink coaches to Campbeltown and Oban also pass through Inveraray. They will get you there more cheaply than a tour.
Please let us all know whether you decide on a trip from Glasgow to Inverary or, on your other thread, about Rosslyn Chapel.