Hi,
May you be great.
First don’t get intimidated about intellisense list of errors, most of them means nothing when it comes to Unreal (an simple refresh/regenerate Visual Studio project files may consume most of them), actually care about compile errors output.
Second, check your your_project.Build.cs if have added the required modules like GameplayAbilities, GameplayTags and GameplayTasks.
Also make sure you have enabled GameplayAbilitySystem plugin in the Editor.
Finally it’s a good practive refresh/regenerate your Visual Studio project files, after that you should be good to go, so press Ctrl + F5 on Visual Studio and enjoy.
Hope that helped you.
Bye.