How to get the symbol name for a memory address in GDB?

I believe you’re looking for:

info symbol <addresss>

Print the name of a symbol which is stored at the address addr. If no symbol is stored exactly at addr, GDB prints the nearest symbol and an offset from it.

Example:

(gdb) info symbol 0x400225
_start + 5 in section .text of /tmp/a.out

(gdb) info symbol 0x2aaaac2811cf
__read_nocancel + 6 in section .text of /usr/lib64/libc.so.6

You can read more about it here.