Have played with this for a while now, and very quickly got my first Google spreadsheet to appear on my website.

The trick is to use the 'key' element given when you 'publish' your spreadsheet rather than the whole URL - just copy and paste it into the 'Spreadsheet URL' parameter when creating your Joomla! module.
Note though that I can only make it link to the first sheet in my spreadsheet, so if you've only got one sheet and you want to display it all then this works fine.

After much trial and error and trying to find out what was happening, I've not worked out that I can specify a range on the first sheet to look at. Populate the optional Spreadsheet Range parameter with the range you want to display, but here's the bit that took the working out - it doesn't seem to recognise the : in the range address - for A1:D10 you need to enter A1%3AD10 replacing the colon with its char equivalent (I think)

I've still not managed to display from anything other than the first sheet, I would suggest the next improvement is to provide a parameter for the GID generated in the link so you can easily get to that, but I've created a workaround by having a number of individual sheets for my data with cells on the front page linking to those pages, and then using the methods above to display them.

So I had a few hours play with this extension. It installs good, and the most parts is working as it should, but the lack of proper documentation makes it hard for even a seasoned developer to use this extension. The few documents I tried to link into my site either gave me error 404, or an error that the spreadsheet was not shared. So I cannot give anything else but a fair vote for this tool which with the proper documentation would become very useful in the right hands. Right now it ain't very useful. For me at least.