What does this Gil Scott-Heron line mean: "God's hole card has been thoroughly piqued."

Language in poetry simultaneously says multiple things at once. Scott-Heron is playing with word pairs, double meanings and polarities that flip from one meaning into another. The focus of the listener must be loosened to free associate along with the poet, yet kept tight enough to sense what the overarching theme might be.

There may be some card game references related to the descriptor, piqued, but it seems to me that the main meaning of the verb pique here is what @Phil Sweet suggests, i.e. to pierce or prick with holes. God's hole card is full of holes rather than being holy. I think pique may also play with peek, as you and @Xanne suggest. We have peeked at God's hole card and rather than milk and honey in the Promised Land, we've seen blood and tears. The whole thing is a game. Living is upside down. A preacher's sermon -- maybe even one by Martin Luther King -- isn't enough. It's just talk. Now is the time for action.

After all, by 1970 so many people are dead. including King. I don't know Scott-Heron's work, but I think he must be making reference to the "I have a dream" speech and the nightmare of King's assassination that followed. Note the last lines:

The whirlwinds of revolt will continue to shake the foundations of our nation until the bright days of justice emerge.

In the way of poetry, it is possible for a card to be peeked or piqued and to mean multiple things. You and @Drew tell us that pique in French is the card suit spade. Spade of course was a term for African Americans that came out of the counterculture of the 60's. Spiel is German for play, but it's also a bunch of fast talk. Spill is to tell the truth in an outpouring, but it also reminds me in this poem that blood can spill. It may not be so important to get the exact meaning, as to swim in the double-ness. Duplicity can trick and fool us and later reveal a hidden polarity. Perhaps your lack of sureness about meaning is precisely what Scott-Heron meant to evoke.

I think Scott-Heron is saying instead of playing games and watching things go upside down or observing the upside-down-ness that has always been there, the kind of upturning that is needed is revolution.

See pique at Free Dictionary for a wide range of meanings