Do you have a working version of Gallery on your site? That's the only way I know of to integrate Gallery. The code (for Geeklog) has to be accessible to Gallery's code, otherwise it wouldn't be able to validate Geeklog's users and put up the Geeklog headers/footers.
I'm not sure if the data for Gallery can be offsite or not. That may be possible, if you have it set up correctly. That's more of a Gallery question. But the code portion of Gallery pretty much has to be local for it to operate as a plugin.