The error is pretty much self explanatory: your code will not compile becuas ethe class or namespace "GanttProvider" cannot be found.
Start by looking at the error message carefully - it will give you which file and line the problem was found in (this may help you understand the details it gives you:
How to Write Code to Solve a Problem, A Beginner's Guide Part 2: Syntax Errors[
^]
When you know exactly what it is looking for, then add the appropriate
using
statement(s) to each source file that references it, and / or add the requires reference and make sure the assembly is installed on the server, or available in the folder.