Just Found Out How To Make That Grungy Grainy Or Noisy Looks in Your Gradient Like This

Just Need AE (After Effects) 📽️

Ctrl + Y | CMD + Y ⌨️

Go To Effect > Noise And Grain > Noise > Set Amount To 100% > Then Turn Off Color Noise

Duplicate The Effect If Want More Darker 🕶️

Right Click The Main Composition > Composition Settings > Set Duration to 0:00:00:02 (2 Frames)

Then Hit "OK" > After That Ctrl + M | CMD + M Click On Output Module > Select PNG Or JPEG Sequence > Hit OK Again > Hit Render

Go To FlutterFlow Add Background Image To Your Container The Exported PNG Or JPEG Will Be In A Folder In Your Project Export Folder! 🎉 🎊 🎉