Thank you so very much for your help, I seem to be making progress. Evidence suggests that Homebrew successfully built GDB, but I still can't find the executable, therefore can't tell Lazarus where it is. Evidence:
When I type GDB in a Terminal window, it responds as follows:
Stephens-MBP:gdb-8.1 stephenrooney$ gdb
GNU gdb (GDB) 8.1
Copyright (C) 2018 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-apple-darwin17.3.0".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word".
(gdb) q
I took this to mean that it has been built, but is hidden somewhere. I think I am almost there, just need one (or many) more pointer. Not the Pascal Pointer type, which should only ever be used by a Handle.
"There are whole libraries full of stuff I don't know."