Conversation so far:
What is Radio Gnome Invisible trilogy?
Flying Teapot was released on 23 May 1973, the same day as Tubular Bells, and was the first installment of the Radio Gnome Invisible trilogy.
What was Tubular Bells?
Mike Oldfield's Tubular Bells.
Was Mike Oldfield part of Gong?
Kevin Ayers, and who had replaced Oldfield in Ayers' band.
How did Flying Teapot do?
expounded upon the (previously only hinted at) Gong mythology developed by Allen.
What was the Gong mythology?
CANNOTANSWER.
What was the second part of the trilogy?
Angel's Egg.

Question: When was Angel's Egg released?
Self-contained rewrite: When was Gong's song Angel's Egg released?