Update doc page

Author: mborland

doc/modules/ROOT/pages/debugger.adoc

@@ -8,15 +8,37 @@ https://www.boost.org/LICENSE_1_0.txt
 = Debugger Support
 :idprefix: debug_
 
-Currently, Boost.Decimal supports pretty printing with LLDB.
+Currently, Boost.Decimal supports pretty printing with LLDB and GDB.
+
+== LLDB
+
 To load the pretty printer, add the following line to your `.lldbinit`:
 
 [source]
 ----
-command script import /path/to/decimal/extra/decimal_printer.py
+command script import /path/to/decimal/extra/decimal_printer_lldb.py
+----
+
+== GDB
+
+To load the pretty printer, add the following line to your `.gdbinit`:
+
+[source]
+----
+python exec(open("/path/to/decimal/extra/decimal_printer_gdb.py").read())
 ----
 
-Once you have loaded `decimal_printer.py` you can run the following example to see how different values are represented with the pretty printer.
+or you can source it manually in GDB:
+
+[source]
+----
+(gdb) source /path/to/decimal/extra/decimal_printer_gdb.py
+----
+
+== Example
+
+Once you have loaded a pretty printer, you can run the below example to see how different values are represented with the pretty printer.
+This expected output of the example below was taken from LLDB, but the results should be quite similar when using GDB.
 
 .The following example can be run with the debugger to show a variety of values
 ====