In Godot 3.x, you can only use the Godot shader language – you can't import GLSL files directly. This language has GLSL-like syntax, but it is not GLSL.
In Godot 4.0, you can use GLSL files directly, but only for specific advanced uses (it's not intended to be used for material shaders). For most use cases, the Godot shader language is more suited.