Gasser \GAS-er\ , noun;
1. Something that is extraordinarily pleasing or successful, especially a very funny joke
2. A person or thing that gasses
Gasser dates to 1829 as "a person responsible for gassing yarn or fabric." The slang meaning of "a talkative or boastful person; a chatterer" is first attested in 1855 and presumably based on the US slang word gas or gasbag. Both words are first attested in in the 1840's and are either the act of or the person who "talks at length but says little of value."
The Oxford English Dictionary does not list the first definition above. I would guess that meaning to be based on the expelling of air that happens when you laugh.