GDB is the GNU debugger, it's available for UNIX-like systems, including Linux.
Valgrind is also available on Linux machines at large.
help50 and debug50, however, are CS50 IDE specific.
debug50, as far as I know, is a wrapper around GDB that allows for visual debugging (as in setting up breakpoints by clicking at the line rather than setting them from the command line, along with some other cool features)