Question about free energy tutorial

GROMACS version:2021
GROMACS modification: No

I have read and run the free energy tutorial. I am not a molecular expert, but I would like to study some performance tests on GPU. So, my specific question is, how can I replace the current input with a larger one?

When I run the current input, Methane in water, only 200MB of GPU memory is used. Below, I would like to share my thoughts and see if I have understand the structure correctly.

1- In methane_water.gro, the number of atoms is 1793. If I want to use a larger input, I expect to have a .gro file with a larger number of atoms. Is there any repository for such .gro files?

2- Looking at the topology file, I see

[ molecules ]
; Compound             #mols
Methane                1
SOL                    596 

I would like to know if increasing the number of Methane or SOL alone is sufficient or there is a relation between these two parameters.

3- I know that based on the numbers in topol file, 1 and 596, atoms are defined in the coordinate file, .gro file. So, I was wondering how can I correctly enlarge the input.

Any feedback is appreciated.