Hello Tadahiro Takase:
Those example projects have relative include paths. It seems you are copying the project instead of importing the original, so the relative paths are not valid anymore.
To solve the problem do this:
1) Delete the project from your workspace:

2) Import the project again, but uncheck the box "Copy projects into workspace":

Now you can build with no errors and also you can open GPIO_demo.c.
Regards!,
Jorge Gonzalez
-----------------------------------------------------------------------------------------------------------------------
Note: If this post answers your question, please click the Correct Answer button. Thank you!
-----------------------------------------------------------------------------------------------------------------------