A cdbaby product page has a 12 digit number (194171346972) on it.
And it looks like cdbaby is (at least now) demanding a barcode for products submitted by Artists:
Is the 12 digit number a barcode?
(Google finds it unique to the top above cdbaby URL)
UPC, JAN, EAN, UPC: All of them are GTIN.
I came here to second this.
I’ve seen UPC codes beginning with 1 on some newer physical releases as well.