How to get the DC gain of an active high pass filter?
The DC gain of that circuit is zero. Once the capacitor impedance drops significantly below R1, the gain becomes -R2/R1. That only happens when the input frequency is significantly greater than \$\frac{1}{2\pi f R_1 C}\$ as semi-alluded-to in the picture. Not all web resources are good at explaining everything.

I feel like that would mean that the DC gain is 0V

Gain is not defined in terms of units because it is volts per volt.