Gate ! does not own or operate the river cruise ships. Gate 1 is simply an online travel agency. Check to see what company acutally owns and operates the cruises you have in mind. (My TA might know if Gate 1 won't tell you.) I did two Danube cruises on Viking and they were very nice. Viking is a good company, but I really like Amadeus Waterways for Europe (free wine with dinner, generally newer ships).
As for ownership, many airlines lease their aircraft from giant leasing companies. Even though the aircraft is paintrd in United Airline colors, there is over 50% probabilty that United has leased the aircraft. This is true for almost all new aircraft models. When the lease is up, the aircraft is flown to Arizona for temporary storage, until needed again, and to be repainted if a different airline is leasing.
We returned on Nov 17th, after a Danube cruise on Gate 1 "Amadeus Symphony". Everything was great, except the weather. Mostly cloudy, with rain in Salzberg. Cabin was clean. Food and service excellent. Only 84 passengers instead of 146 max. All tours were excellent. Petr (27-year old Czech) was our guide on the Yellow bus, from Budapest all the way to Prague (transferred from ship in Numerberg to Prague by bus). I highly recommend Gate 1 Danube cruising. We are going to Turkey with Gate 1 in Feb 2009.
I returned Dec 12 from the Christmas Markets on the Rhine with Grand Circle. We went from Basel, Switzerland to Amsterdam and did the 3 day pre trip in Basel. This was my second trip with Grand Circle and they did another excellent job. I really enjoy that so much is included with the river trips and all the personal service we got!