Gaussian Blur in my custom menu

[nukiz]

I am continuing a discontinued mod, and I've had success for some time now. There is one issue: I wish to remake the blur in it's custom GUI, because it looks a little weird, and not the one I wish to create.

I want to make a gaussian blur, instead of a more linear-ish blur, which the mod currently has. How would i go on about creating something like this? 

[ChampionAsh5357]

Probably with a shader. Minecraft should already have one iirc called blur.