Hello Mr Seung!
Good day to you!
I’m Daniel Tan from Huttons.
Hopes the below information helps.
Reference from: http://www.nus.edu.sg/uci/SPM/SpacePlanning/TermDefinition.htm
GFA is defined under the URA guidelines as measuring all sheltered floor areas of a building and unsheltered areas for commercial uses for purposes of plot ratio control and development charge.
AFA refers to the total amount of space within a building, measuring from external wall to external wall that can be assigned and available for the occupant for specific uses.
NFA refers to the space inside a building, measuring from external wall to external wall excludingbuilding circulation and service areas such as restrooms, elevators, vent space and stairs. Thus NFA is the space that is available for occupant allocation for specific uses.
Reference from:
http://www.nus.edu.sg/uci/SPM/SpacePlanning/TermDefinition.htm
Any other queries?
Please feel free to contact me for a non-obligated discussion on how I can assist you with your real estate needs.
Have a great day ahead.
Best Regards
Daniel Tan
Huttons Asia Pte Ltd
(+65) 94775100 daniel.tck@outlook.com
Read More 
Then, what is the difference between built-up area and Net Floor Area?