Does gcc define anything when -g is specified?

Running
g++ -E -dD -xc++ /dev/null
vs.
g++ -E -dD -g -xc++ /dev/null
shows me no extra defined symbols.

That, combined with the lack of any documentation stating that symbols get defined, should be enough to safely say that nothing gets defined.

As others have said, and as is apparent from the manual, GCC does not provide any intrinsic macros that indicate whether debug information will be generated (-g mode). What I want to add, is that this is on purpose.

Long, long ago, the original authors of GCC (RMS, Kenner, probably a couple others) decided that enabling debugging information should not cause any changes whatsoever to the actual code, to reduce the risk of a bug mysteriously vanishing when you try to debug it. This is a bedrock design principle in GCC, and even today the developers go out of their way to maintain it.

Defining a macro would violate this design principle, since it would permit the source code to change itself in response to -g.

You can see the list of all macros gcc/g++ defines for any combination of flags like that:

$ g++ -E -dD -xc++ /dev/null

For example:

[max@truth ~]$ g++ -E -dD -xc++ /dev/null > a
[max@truth ~]$ g++ -E -dD -xc++ -g -O3 /dev/null > b
[max@truth ~]$ diff a b
1a2
> # 1 "/home/max//"
173c174
< #define __NO_INLINE__ 1
---
> #define __OPTIMIZE__ 1

Let's see whether -g defines anything:

[max@truth ~]$ g++ -E -dD -xc++ -g /dev/null > c
[max@truth ~]$ diff a c
1a2
> # 1 "/home/max//"

Extra preprocessor directive, but no extra macros for -g flag.

---

This makes sense: -g has no effect on code-gen, only on object-file metadata (debug symbols).

-O0 is "debug mode" code-gen for consistent debugging, as opposed to -Og or higher. (This answer explains what that means in more detail for compilers like gcc and clang: store all variables back to their memory locations after every statement, and don't optimize across statements.)

-O3 and -g are orthogonal (except that you get "optimized out" for some variables when debugging optimized code, and execution appears to jump around between source lines.)