Go to your GD Launcher settings on the top right.
Within the GD Launcher settings go to the Java tab on the left side.
Look for where it says "Java Memory" there should be a slider that you are going to use to set the ram your launcher will use for your game.
The amount of ram you want to set for your game will depend on the total max available ram your computer has. Windows needs a minimum of 2GB of free ram to function properly, so do not set all your available ram to the game. The minimum ram most mod packs need to run without lag or crashes is 5GB.
This is what we recommend you set your ram to if your max available ram fits the following criteria.
8GB or less
Recommended ranges to set ram to 5GB - 6GB
5GB = 5120MB
6GB = 6144MB
12GB or greater
Recommended ranges to set ram to 8GB - 10GB
8GB = 8192MB
9GB = 9216MB
10GB = 10240MB
For those of you with 12GB or greater setting to much ram to Minecraft can cause more lag and its not recommended to go above the ranges that we gave.