One trick I use to format embedded content, including an iFrame Google Doc, is to use the Block Library’s Audio and Video block type, and then replace the [embed] code for the video with the iFrame.
Either way, you’ll need centered section of a block to center your iFrame. Here’s the basic HTML you’ll need:
<div class="col-md-12 col-xs-12 col-sm-12 text-center">
<iframe src="https://docs.google.com/document/d/e/your-embed-code/pub?embedded=true" width="640px"></iframe>
I found that I needed to specify a “width” property for the iFrame, by default it was very small. You can change the width of 640px to suit your needs.