Oh dear. A slightly embarrassing misreading. Lief Ericson was converted to Catholicism at the court of King Trygvasson of Norway, and founded Gardar, Greenland. When he founded Gardar, he immediately sent for two priests. Gardar became a diocese in 1112, a full century after its founding. Gardar was founded by Ericson; the diocese of Gardar was founded by Eric. I misread, “The Catholic church had, in fact, established a diocese of Gandar, in North America. Gandar had been founded by Lief Ericson” to mean that the diocese of Gandar had been founded by Lief Ericson. That may seem like quiute a silly leap out of context, but keep in mind that in context, Lief had been a First Convert, brought priests under his authority, etc, etc, etc...
Better a quickly tracked down mistake than a new urban (?) legend.