This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. The 国 is changed to the reading ごく, though because of rendaku. Even heaven changes a little bit.
Poochie is definitely in heaven.
I am so disappointed to learn the Kingdom of Heaven is that kind of place.
Let’s smoke cigarettes in Heaven.