I think the above three lines would be better reworked as:

Then start GDB. *Before* connecting to an Opella-XD target it is essential to specify the architecture of the target. For EM use

  set arc opella-target arcem

(other possibilites are arc600 and arc700).

Then connect to the target as with the OpenOCD/Linux gdbserver. For example a full session with an Opella-XD controlling an ARC EM target could start as follows: