Having solo strings with controllable vibrato is one of the things being worked on for a GPO4 upgrade. One version was tested before the release of the current GPO, but it was decided more work was still needed before it could be released.
Like other orchestral libraries, the string samples are recordings of performers playing the instruments with vibrato, which of course is the usual way of playing the instruments. Since the vibrato is part of those recordings - there's no way to turn it off - it's set in the stone of the recordings.
GOS is a now discontinued Library, Garritan Orchestral Strings. It was a high end product costing 10 times more than GPO, but with the kind of flexibility typical of high end libraries, including string harmonics. GOS is going to be revived and released at some point as GOS2 -something to look forward to.
good to hear ...
I am aware that it is a question of personal taste.
The sound of strings without vibrato has got
a very special quality which I personally like
very much and it would be good to have it.
So let's wait and see ...