Here's how you go about inserting it in Google Sites:
Click [Insert]
Select [More...]
Click [add by url]
Copy and paste the url to http://grinsted.googlepages.com/flexiblegadget.xml
Note, there has been some bugs in Google-Sites which means that preferences of the gadget is forgotten. One trick to overcome those problems is to insert the gadget in an igoogle page, set up all the settings, and then inspect the source/dom to find the relevant iframe link. This link can then be copied and put in place of the Google-Sites generated iframe link (by entering the html mode of the Google-Sites editor).
Perhaps too complicated, but this is a detailed guide for gadgets.
