If you're already using Squarespace for your website, you can use Gist on it as long as you have a custom domain on Squarespace.
That means, you cannot use Gist on your Squarespace website if the URL looks like this: http://example.squarespace.com/
In order to get Gist to work on your website, you would need to make sure that you get a custom domain for yourself, which looks like this: http://example.com
To get started, login to your Squarespace admin, and click on 'Edit'.
Then, click on 'Add Block' or the '+' sign and then add a new 'Code Block' from the options menu.
Once you do this, the Gist tracking code will be installed on your Squarespace website. You can start tracking visitors and their actions on your site, and also talk to them in real time using the Gist live chat messenger.
For more details about installing custom codes on your Squarespace site, check out their in-depth guide here.