Why are some Gamedonia classes not found? [Unity]

Before calling any Gamedonia methods or declaring Gamedonia variables (like GDUserProfile) it's important to check that you imported the Gamedonia SDK .unitypackage file into your Unity project. You can see how to do that in our Get Started guide for Unity.

If the SDK is not there, you will get errors like this:

Assets/Samples/Login/Scripts/UserDetails.cs(11,17): error CS0246: The type or 
namespace name 'GDUserProfile' could not be found. Are you missing a using 
directive or an assembly reference?