G fixes some pathfinding issues and a problem with submitting metaverse games after re-registering your serial numbers, and maybe a few other things. I think that I put up a change log at http://carielf.joeuser.com which gets posted under the journals section here as well.
You can avoid a re-install by changing the version string in the registry.
First, close Stardock Central. Go to Start->Run and type in regedit. On Vista, type it in the search box. As soon as the registry opens, click on the My Computer icon and choose File->Export to backup the registry.
The install info for GalCiv2 is under HKEY_LOCAL_MACHINE, in the path Software\Stardock\Drengin.net\GalCiv2\DarkAvatar.
Just change the version string to 1.7 and SDC will show an update to 1.80g.
Befiore installing the update, go to Tools->Settings and click on Internet. US-1 is usually the best choice for servers because sometimes we have sync errors with US-2. Click Refresh XML Data. Also, if you have the setting turned off to check for updates to Stardock Central, turn it back on and then re-start Stardock Central. If Stardock Central finds an update, let it update then restart SDC when prompted.
If the above method doesn't work for you, you should probably e-mail support@stardock.com as you may be having a more general issue with downloading from our servers.