You can safely assume that a directory can be deleted when the instructions tell you to reuse that directory.
If you don't feel comfortable deleting the directory without the book explicitly telling you to, the scan back over the sections. There is likely to be a footnote or a short sentence that tells you the gcc-build directory is no longer needed. If you can't find it and are still hesitant to delete the directory, you have two options:
1. Create a different directory. Instead of gcc-build, name it gcc-build2, gcc-build-second-pass, or whatever you like. Just adjust the command in the book appropriately (which amounts to little more than the mkdir and cd command)
2. Rename the old directory. Just do a "mv gcc-build gcc-build-first-pass" or something similar. Then you can use the book's commands exactly as written.
And you are more than welcome for the help