I want to record my voice for YouTube videos. So should I get a headphone with mic like
Senheiser GSP 300 or a microphone separately and a monitor headphone..?
The quality of the headphones will only affect how the audio sounds to you, so you don't necessarily need to spend your money on that to improve the audio for a youtube video. I would suggest focussing your money on the mic itself.
Gaming headsets tend to have relatively cheap mics in them, and you need to be really careful about the mic's position. Too close and you get breath noise and popping sounds. Too far away and you get room noise and hissing on the audio.