In the presence of closed-source drivers that manage them and compile the shaders? I'd say probably not. Something open-source with actual direct access to the opcodes (would Mantle work? Intel's embedded GPUs?)… maybe. I don't know if I'd consider them secure. Some running hot and loud and by the seat of their pants? The presence of DMA? Hm. I have my doubts they'd be better than the CPU safety-wise.

What I do know is they can usually run them very fast if asynchronous low-communication parallelisation is not undesirable - GPUs overtook PlayStation 3 Cell processors for the "throw watts at it, but we don't have enough money to burn FPGAs or ASICs" class of crypto attacks quite a few years ago now. (As anyone who runs a Bitcoin mine knows!)

Might be effective on, say, the Raspberry Pi, where the "GPU"'s the ringmaster and the ARM's the clown. That vector processor looks tempting, and if I could figure out how to get it to do diagonals, it's a poster child for ChaCha.