Gamma color looks as if the image was processed to add more contrast, which is almost what happens unintentionally: Texture contains compressed color to save space, you monitor decompresses it to show you the correct colors. But shaders use that compressed color, multiply it by light and, sometimes, another compressed texture, like detail, color can be averaged with pixels next to it, and the result becomes incorrect.
It’s not too incorrect since most games used and are still using gamma. Switching to Linear workflow basically, means that every color will be decompressed before processing = some performance hit. But linear is the correct way, colors will look more natural, and it becomes more important in complex shaders.
You can enable Linear workflow in Unity's Build Setting. But keep in mind that mobile platforms and HTML5 do not support it. Unity made it very convenient to use both, Linear and Gamma workflows can be interchanged in Build settings.
Also, take a look at the video below for a great explanation.