I’m not certain what the devil has got into some writers, but there’s a lot of confusion and inconsistency regarding this phrase.
Part of the problem is that there are a number of regional variations in its use. Given this, God’s sake, god sake, and god sakes all are correct.
The odd man out (gods sake) is wrong because God is not capitalized and the apostrophe is skipped. In this specific instance, God is a proper noun, referring to a specific supernatural being, and sake belongs to God, requiring than an apostrophe to show possession.
So for Heaven’s sake, start capitalizing and punctuating this phrase correctly!
My name is Rob Bignell. I’m an affordable, professional editor who runs Inventing Reality Editing Service, which meets the manuscript needs of writers both new and published. I also offer a variety of self-publishing services. During the past decade, I’ve helped more than 300 novelists and nonfiction authors obtain their publishing dreams at reasonable prices. I’m also the author of the 7 Minutes a Day… writing guidebooks, four nonfiction hiking guidebook series, and the literary novel Windmill. Several of my short stories in the literary and science fiction genres also have been published.