Unless I am overlooking something, there is currently no way to serve native/responsive ads from Google Ad Manager. The only options are for fixed sizes.
The line in the Google Publisher Tag looks like this for native/responsive only:
googletag.defineSlot('/123456789/ad-unit-code', ['fluid'], 'div-gpt-ad-987654321-0').addService(googletag.pubads());
And looks like this for mixed sizes including native/responsive:
googletag.defineSlot('/123456789/ad-unit-code', [[320, 50], 'fluid', [120, 90]], 'div-gpt-ad-987654321-0').addService(googletag.pubads());
I know I could manually put the full tags wherever I want with theme components, but then I wouldn’t get any of the other plugin features like no ads for certain groups, etc. I would be hugely appreciative if this size option could be added to the plugin.