After have Postgres configured with enabling debugging flags and a running Postgres instance. An open session running and age extension is loaded

• Get PID of the current session

SELECT pg_backend_pid();

• Open new terminal tab and run sudo gdb to run the debugger
• Attach gdb to to the PID that we got from the previous step e.g. PID: 123

(gdb) attach 123

• Add a break point to a specific line or function at some where

(gdb) b <function_name>

The following are The basic GDB commands:

• b for breakpoint, (b )
• c for continue - continues to the next breakpoint
• n for next line
• s for step into
• p for print, (p *) for pointers
• d for delete all breakpoints
• q for quit

Ref:
apache age repo
GDB Docs