Comment by kafrofrite

You can target co-processors in general, e.g., here [1], thus I assume people do hack GPUs.

Generally, the better we become in introducing mitigations, the more expensive attacks become and attackers have bosses, budgets and deadlines. They will try to find other avenues to land on a target :-)

[1]https://objectivebythesea.org/v5/talks/OBTS_v5_iBeer.pdf