How to dump all the XMM registers in gdb?

(gdb) apropos registers
collect -- Specify one or more data items to be collected at a tracepoint
core-file -- Use FILE as core dump for examining memory and registers
info all-registers -- List of all registers and their contents
...

The last one is the one you want.

See also this answer below.

The fine manual says:

(gdb) info all-registers