Find the path of the source code for the executable being currently debugged in GDB

You can use the GDB command:

info source

Sample output:

Current source file is a.c
Compilation directory is /home/user/test
Located in /home/user/test/a.c
Contains 17 lines.
Source language is c.
Compiled with DWARF 2 debugging format.
Includes preprocessor macro info.

Use

(gdb) show directories

If you don't know where those directories are set, check your .gdbinit file to see if there are statements like

directory /path/to/source

See also this other Stack Overflow question about GDB.

This information is kept in the binary in the DWARF2 format. So, in order to see the DWARF2 information, you can use the dwarfdump utility. Needed information is kept in the DW_AT_comp_dir field.