Part of the math here is that you state you have 6Gb of ram.
If that is right then by the time you load windows that sits in system memory all of the time, using up on average about 1.5 to 2Gb.
Leaves you with free system memory for other programs, or games to run with 4Gb.
To check this, what is free after windows OS has loaded into system memory.
Click on start.
All programs.
Accessories.
System tools
And system information.
Look at available system memory and tell us what it says is free in Gb.
If it turns out you do have 4Gb free.
Run Gta 5 in a window other than full screen, set to 640 x 480 resolution or 800 x 600.
Follow the same set of instructions to get to system information.
And read off how much system memory is free when Gta 5 is running.