Can someone recommend a gaming headset/headphone under a 100 euros?

You may find this useful:
https/uk.pcpartpicker.com/products/headphones/#X=0,10334&R=5,4&sort=-rating&page=1

You can also google for "best gaming headphones 2017" etc. and read the comments.

Some of the better ones have a DAC (Digital to Analog Converter) and use USB, but ideally the unit is separate. By that I mean it connects via USB to the computer and converts audio to a STEREO 3.5mm headphone. So the headphones can be regular stereo headphones and then connect the INCLUDED DAC if you want virtual surround.

(I do not like headphones with multiple drivers. I prefer a single, larger driver per ear which gives better bass as well)

The DAC can thus create "surround sound" though the sound may be worse in some ways due to the processing but the advantage is ideally a more 3D sound (useful for shooters and other games).

It's nice to have the option of using the headset both ways. (you may have to manually switch "PLAYBACK DEVICES" from the audio icon between onboard audio for the 3.5mm, or USB audio for when using the DAC).

I don't have an opinion otherwise.

OTHER:
Make sure the OHM rating is within the sound card/onboard sound specifications. Doesn't apply if USB (or other digital) only analog connections.