There are Prime-based algos (Rie, Prime, Gap), Blockchain-based scratchpad algos (Wild Keccak) and memory-intensive ones (Cryptonight). Can take a look at http://cpucoinlist.com/. These are all based on high random access memory requirements which GPUs perform poorly with.

These are mostly well-established algos with known optimised GPU miners, actually. But the main difference is the GPU miners don't have much of a Hash/Watt advantage over CPUs even after heavy and public optimisation, whereas for Dcrypt it could be a lot higher and still private.

Alternatively, we can also focus on PoB for the masses, and just use an established Scrypt or SHA256 PoW for miners.