IPA: /ˈɡæsɪli/
KK: /ˈgæsɪli/
In a manner that is characterized by gas or gas-like qualities.
The balloon floated gassily in the air, swaying gently with the breeze.
The word 'gassily' is derived from 'gassy' (from the word 'gas', which comes from the Greek 'khaos', meaning empty space or void) and the suffix '-ly' (meaning in a manner of). Therefore, 'gassily' means in a manner that is characterized by gas or gaseous qualities.
Think of 'gassy' describing something that is full of gas, and the '-ly' indicates the manner, so 'gassily' means behaving or occurring in a way that is full of gas.