It's always nice to answer your own questions... ;-)
First of all, I got it all to work now! :-)
The last bit of confusion was with the groups.
The proper workflow in gContactSync>accounts seems to be:
- select the addressbook,
- link the (Gmail)account,
- use the "Find Groups"button to populate the dropdown menu with the Groups created under the Gmail "MyContacts" section,
- select the corresponding Gmail group from the dropdown menu.
Done.
This took a while to "get" and sink in.... I was still thrown off my the multiple email accounts login errors I guess.
I did some tests with changing/adding to accounts, and so far all seem to work.
Now I have to erase all my contacts from the phoine, and resync after setting it up for the Google account.
Hope it helps someone, I found the installation and first steps simple enough, but linking everything into Gmail and such was a bit confusing.
But happy now, and I'll see how it goes from here

rob