Gas bag = wind bag, pompous, talkative, bloviator. One who bloviates - constantly talking, often in a longwinded pompous manner. Many of these folks like the sound of their own voice - often unaware of those around them who are often laughing at them behind their backs. These folks are often very vain and conceited - impressed by their own self importance. Many politicans are gas bags, full of empty hot air.
These folks are sometimes referred to as popinjays.
Many politicans are gas bags - long winded pompous people who are usually bloviating over their own self importance.