No, you can use it like your box blur. It´s just an other kernel and happily separable too. For a 7x7 blur you can try these weights: float gauss1D_weights2[7]={0.015f,0.083f,0.235f,0.332f,0.235f,0.083f,0.0015f}; but you can compute your own of course, depending on the width and falloff you prefer. Not much effort as you can see.
Just give it a try. You'll notice that your bloom fates out more smoothly.

I've tried to implement gaussian blur, the falloff is really smooth now, but maybe a little too smooth as I want the bloom to stretch nicely and wide across the screen. I will keep experimenting, though (maybe something like "cosinus blur").

I've rewritten the clouds, first screen is without shader:


Second screen is with cloud shader applied (I need to create better cloud models, though):


And the last screen shows the start of a lightning:

Cosinus is most likely not separable. But you can use any falloff you can produce with a Gaussian bell curve.

What do you mean with "not separable"? Well the problem is I don't like the form of Mr. Gauß's curve for a bloom effect.

"Separable" means you can blur in x- and y-direction separatly while (theoretically) getting the same result like filtering directly. O(2n) instead of O(n²), but I guess you know that.

Ok now I get what you mean with separable.
I now have two different switchable blurs implemented, the gaussian blur and one where I simply use the weights {0.25,0.5,0.75,1,...,1,0.75,0.5,0.25} (and divide by their sum). It works quite well with the blocky geometry and both blurs look much nicer than my previous box blur, so thank you very much for the tip!

(double-posting for more attention)

I've just removed a level (where you mostly progress using airscrews/ upwinds) because it wasn't fun, so I'm in need for a new one (or two, if you have good ideas). It has to be indoors and should not contain ice/ snow and little to no lava.
If you have an idea or know a nice level from any side-scrolling game, I ask you nicely to share it with me!

Great
It seems that each level or group of levels will have their own particular ambient touch and effects ?

Sadly I don't have the resources to create an individual look for each level, I'm already struggling a little with the content creation, but I try to give most levels a unique gameplay twist.
With that in mind I'm still looking for a new level idea!

A lush mine? An insanely twisted shadow cave!