Chevy Avalanche vs Honda Ridgeline?

Which one is more gas efficient?

Neither are more fuel efficient according to mpg test done on both cars in all of their models. You can somewhat increase fuel efficiency on both cars by adding a tonneau cover or ride with the back down to reduce drag.

The Honda is a much smaller, lighter weight vehicle. It has a smaller 6 cylinder engine as opposed to the Chevy's V-8 engine, so naturally the Honda is more fuel efficient, HOWEVER, it doesn't have the capabilities of the Avalanche for towing or carrying heavy loads.

Ridgeline. The Avalanche is for dudes who lack something between their legs.

Get it from the horse's mouth. Go to the website indicated and you can do a side by side comparison.