Here are some counters I have found that work with Google Sites
Some counters will not appear if you have an ad blocker in use.






StatCounter 

It is also in the sidebar.
 
web analytics
















ShinyStat
I created a gadget.





StatCounter
Go to http://www.statcounter.com/ This one gives a some stats that you can view.  You can view your stats by clicking on the count and signing in with a password. See their Instructions for Google Sites.

You can't install the entire code in the sidebar but you can put the image there. It will show the count but it will not be clickable like in the main part of the page.

Copy the the code in red from your install code:
<a title="shopify analytics tool"
href="http://statcounter.com/shopify/" target="_blank"><img
src="https://c.statcounter.com/xxxxx0/3f0ab45e/0/"
alt="shopify analytics tool" style="border:none;"></a>

Add a textbox to your sidebar, click insert, select image and use the image URL you copied.