You will need an AArch64 compatible GDB. The versions that ship with Ubuntu 16.04 on AArch64, as well as the Linaro public releases since 7.1.1 (x86_64 Linux) have been tested to work. The AArch64 debug protocol was somewhat in flux until 2015, so very old GDB versions may not interoperate correctly.
If you are using the cloud product, you will need to be connected to VPN.
To connect to the stub:
Note: The address and port provided here are for example purposes only. You will need to use the address and port for your particular virtual device. You can find the address and port for your device at the end of the "kernel gdb" link, located at the bottom of the virtual device page.
(gdb) target remote 10.11.1.1:44219
Remote debugging using 10.11.1.1:44219
warning: No executable has been specified and target does not support
determining executable automatically. Try using the "file" command.
0x00000008030e40c8 in ?? ()
To switch CPUs (in this case, to CPU 1):
(gdb) thread 2
[Switching to thread 2 (Thread 2)]
#0 0x00000008030e40c8 in ?? ()
To access monitor commands (see above):
(gdb) monitor sr ttbr1_el1=0x0000000034d4593d
CPU 1, ttbr1_el1 := 0x0000000034d4593d (before: 0x0000000000000000)
Otherwise, use regular GDB commands to control the debug stub.