The game resolution seems to be 1024x600.
In hex values, 1024->0004 and 600->5802.
640->8002 and 240->F000.
I used an hex editor to open the exe, and searched an offset where 0004 abd 5802 were close, supposing they were resolution related values.
Horizontal Res:
Change 0004 by 8002
001119d0, 001119e0
00111a40
Vertical Res:
Change 5802 by F000
001119e0
00111a60
Once modified the exe with the new resolution, used ahk to modify window style:
WinMove,0, 20,
WinSet, Style, -0xc40000, ahk_exe gs2.exe,
On the other hand, for me, the game seems not to launch correctly in resolutions lower than 640x480.
So I use a resolution changer program to switch to 640x480 before starting the game, and once started, go to 640x240