Error Message: Gallery seems to be inside Joomla/Mambo, but we could not get the necessary info

This problem is because your joomla/Mambo url doesn't match the url that is in your Gallery config file.
For example, the url you are visiting is http://www.yoursite.com and the url in your Gallery config file is http://yoursite.com/gallery

The solution is to update your Gallery config file and be sure to include the 'www'

From Andreas Studer:
Further it's also important to visit the Joomla/Mambo site with the
'www'-URL. E.g. if both configurations have the 'www' included and a
User visits the Joomla/Mambo site with the URL 'http://yoursite.com', he will run into the same problem.

If you are running Joomla 1.0.8 set the "Session Authentication Method" in Joomlas global configuration to "Level 1 Backward - Compatibility".
If you are running Mambo 4.5.3h please read this thread from the Mambo forum .