I discovered something while trying to troubleshoot this plugin yesterday. (Read the whole comment, plugin is still not working properly, but I think I figured out half of the issue!)
When I get the, "Invalid API Key, username, and/or password. Please try another combination", it is when I am using my GravityForms Developer License code for a site that is using a different CC login credentials.
If I use my CC login credentials, no matter what site, there is no error, and I can connect to CC on any site. I am guessing the plugin registers the GravityForms license with the CC API..?
Meaning: It would appear that you have to have a unique GravityForms license code for each install that you are trying to use with this plugin.
However, even when I have a successful connection (integration), data is still not being transfered from my website to CC.