I think that the easiest way would be to export your photos to a folder on your desktop and drag them into the Gmail message template.
You could also try setting your gmail account as your default email account in your browser. So, open your current Apple default Email and do Mail/Preferences/General and set your default email from the settings box that appears. If Gmail isn't presently set as the default you would click on Select and chose your browser (or, if it appears, your Gmail icon) from the Applications folder. If you select your browser, then follow your browser's method of choosing a default email. I'm not sure, but I think that would make Gmail show up in the Share list in Photos. If it doesn't work, you can always change back.
-- Rich