This could be the case, however since we have not been to Garlemald yet, we are unsure either way.
We have seen Garleans both:
Succumb to tempering/imprinting, as well as fall to the influences of the Darkness (CT storyline). If the echo really is hydaelyn's form of tempering/gift, then a Garlean could obtain it as well, probably.
We have also seen Garlean pureblood defectors. I do not see why a Garlean could not be chosen to have the echo based on these criteria
However, as said before, this item would not turn the PC into a real Garlean, it would just be used for roleplay which is usually separate from the MSQ events, or to be used as a forehead decoration/gem