How to create biphase system in Gromacs?

GROMACS version:2018
GROMACS modification: Yes/No
Here post your question
Hi all,
I am very new to gromacs can someone help me how to create 2 phases system in gromacs and to calculate interface surface tension

You can use Packmol

I have used it many times in the past to create an oil-water interface and it worked perfectly

This can also be done entirely within GROMACS: Building Biphasic Systems

Teslim, can you help me in creating water-air interface and calculating its surface tension.