I think that perhaps a simplified example will help. Let's say I had a survey called "PizzaStudy" at www.mywebsite.com/login.html (assuming v8 of the software), and my panel provider wanted me to return back to them PanelID. I would set up PanelID as a pass-in field (or password field if you wanted people to be able to restart if they leave). Then the link I would give to my panel provide would be www.mywebsite.com/cgi-bin/ciwweb.pl?studyname=PizzaStudy&PanelID=XXXX
The Panel company then does the same thing with you and says to send completed respondents to www.PanelCompany.com/complete.aspx?PanelID=XXXX. For you to dynamically fill in the XXXX with PanelID, you would put PanelID=[%PanelID%] in the link. The scripting tags tell SSI Web return the value of PanelID saved to that respondent record.
As an example, if my Panel ID is 1234, the panel sends me this link:
When I'm done with the survey, you send me back to www.PanelCompany.com/complete.aspx?PanelID=1234.
If you have multiple links based on "gid" then you could put multiple terminate/link pages at the end of the survey and then skip logic on the last question determines which terminate/link they go to.