How can I reinstall gcc?
Is Xcode installed in the normal location? (i.e. in /Applications?)

What do you get back from,

$ xcode-select --print-path


Can you see gcc at the following location,

$ ls -l /Applications/Xcode.app/Contents/Developer/usr/bin/gcc
-rwxr-xr-x  1 root  wheel  18560 16 Mar 00:31 /Applications/Xcode.app/Contents/Developer/usr/bin/gcc


If the path from xcode-select is not pointing to /Applications/Xcode.app/Contents/Developer then try forcing to this location,

sudo xcode-select --switch /Applications/Xcode.app