Use Xnview to add an IPTC 'Caption' to each image: it will be shown along with each image as the main descriptive text. Do NOT use double quote (") inside Captions: will cause Gmap jscripts to fail while centering on the GPS coordinates (sorry!).
Download and install latest Google Earth
Use Picasa2 to 'Add geographical label using Google Earth' (from 'Tools' menu) to each picture you want to add to your gallery. This will add new EXIF GPS tags to your pictures, without altering any other metadata.
Download this template (made of some .php files and three subfolders) and upload it on your website, renaming it with the name of your gallery.
[The template is a dynamic .php rivisitation of a static .html template provided with Xnview]
Use Xnview to create a 'Web Album' from your local picture repository.
Be care of keeping the deafault "t_" prefix for the thumbnail file names.
Put the thumbnails created by Xnview in to the 'thumb' folder of your online gallery and the bigger pictures in the folder named 'original'
Make a link to the page named thumb.php and... ...that's all!