BLAKE & GWEN
UNABLE TO WED
Blake Shelton and Gwen Stefani’s duet “Nobody But You” paints a portrait of two people who are blissfully in love — but in real life, it’s been more complicated. Shelton, 43, who was previously married to fellow country star Miranda Lambert, is itching to say “I do,” but Stefani, 50 — a devout Catholic who was previously married to Bush frontman Gavin Rossdale — is adamant about “waiting until the Vatican issues a decision on an annulment,” says a source. (Because she and Rossdale, who have three boys — Kingston, 13, Zuma, 11, and Apollo, 6 — are divorced, the pop star is currently unable to marry in the Catholic church.) “They do have a plan in place,” which includes moving into a new…