I also think it's a burp, for no reason other than "geppu" means "burp." The sound effect is a generic explosion, so that doesn't help, and the artwork isn't exactly clear about which it is. If you can show otherwise you can change the tags back.
Burps can also reek, especially beer/alcohol burps, and the smell doesn't dissipate quickly in the way farts do. (Methane breaks down rapidly in the presence of oxygen.)