There is a casing bug in the Api Explorer for
Gift Cards → Retrieve Gift card from GAN:
the correct method call is
$body = new \Square\Models\RetrieveGiftCardFromGANRequest();
not:
$body = new \Square\Models\RetrieveGiftCardFromGanRequest();
Thanks for raising the issue. The team is working on this. 
This is still incorrect in the API Explorer, took me a bit to figure out why my production site was fatal erroring, but was fine in my local.
Checking with the team to see where they are on this.