Setting up TagTray on your Squarespace site is very easy following these steps.
The first thing you'll need to do is to load the global TagTray includes into your site by going to Settings -> Code Injection -> Header and importing the libraries as documented in the installation docs here.
Adding A Gallery to Any Page
To add a gallery to any of your pages (including product pages), add a Code Block to a page using the instructions found here: http://help.squarespace.com/guides/about-the-code-block.
Be sure the optional "Display Source" checkbox is NOT selected.