What I have done with flute is to set polyphony to 2, set note length to 103 %, and used the pedal. Seems like it ought to work with the strings also. I have forgotten what I did with strings, as most of my things are piano or organ.
It's been quite a while since I've been able to use GPO. In the options, which setting gave strings legato? Neither settings seem to make a difference.
Sorry, my manual is at a friends.
One thing I have noticed is that velocity has a lot to do with achieving legato in the strings. It makes sense, I suppose: a strong attack on each note wouldn't give a very legato sound, regardless of pedal settings, or some other way os sustaining. Also, I am using this through Sibelius, so a slur marking kicks in the MIDI setting for legato sustain ( I can't remember offhand which ~Cx it is ).
Hope this helps.