I was looking up info on GDB's .gdb_index section today and found that it has been superseded by the DWARF 5 .debug_names section. gdb ships a helper tool (gdb-add-index) that can generate and add a .gdb_index section to a binary and in recent versions (I'm using gdb 8.3 on Ubuntu) it can instead generate .debug_names:

$ gdb-add-index -dwarf-5 hello
$ objdump --wide --section-headers hello- | grep debug_names
 32 .debug_names       000001b8  0000000000000000  0000000000000000  00003847  2**0  CONTENTS, READONLY, DEBUGGING

The .debug_names section format is specified in the DWARF 5 spec section 6.1.1, "Lookup by Name".