Hey,
I know the question is a bit old but it does come up quite often.
For this it is best to use Google's built-in option to create an iframe:
File > Publish to the Web > choose the Embed option and copy the code. This will give you a code that looks like this:
<iframe src="https://docs.google.com/a/....../pubhtml?widget=true&headers=false"></iframe>
This won't look great with the automatic height and width so you can add these in:
<iframe width="100%" height="500px" src="https://docs.google.com/a/.../pubhtml?widget=true&headers=false"></iframe>
Hope this helps someone else!