I believe the reason for this is that it's one of the simplest card numbers that passes the Luhn check:

http://en.wikipedia.org/wiki/Luhn_algorithm

Many (most?) CC processors use that number, as it is easy to remember and passes the Luhn checksum.

Pretty sure it's because Gumroad uses Stripe. Heard that from a rival CEO though... So don't quote me.

there isn't many test numbers if I can remember correctly. 4111-1111-1111-1111 is another.

Oddly enough Stripe won't accept that as a test number.

Here's the full list of cards we let you use for testing, including some that trigger specific responses:

https://stripe.com/docs/testing#cards