I forgot to give you the main answer to your question.
To add a gallery to your page click the HTML Fragment tool (near the bottom
of the tool menu at the left of the Publisher web page editor) to draw a box
of suitable dimensions at the place where you want the gallery to appear.
This will open a dialog box in which you will copy the gallery script you
have downloaded from the web.
Depending on the particular gallery you have chosen on the web, you may also
need to manually insert in your website's main directory additional required
files such as css or js documents. Read carefully the instructions given on
the gallery web page.