First of all, what are your settings under: Tools > Options > Debugger
"debugger type and path" (dropdown and edit)?
Is your Lazarus a new install, or an update?
While gdb can be used as debugger (if you actually build and signed gdb), it is not the recommended one.
The recommended settings are:
"lldb (with fpdebug)" for the drop down
/usr/bin/lldb for the edit field