Black and gray tones appear as blocks? Is the source material junk, or is my screen faulty?

Hello everyone! I have noticed something on Netflix, YouTube and other streams that I think is worth mentioning: My monitor (Gigabyte G27Q) displays black and gray tones only as "blocks." Bright areas are displayed perfectly. This effect only occurs when streaming or watching YouTube. When gaming, the image is perfect! So is it the source…