plot the max slopes of Voltage-Transfer characteristics in ngspice
Using deriv means differentiating across all the length of the trace, so your command .meas g deriv v(out) doesn't make sense, since the return value would be a vector of points. The manual describes the usage of deriv as a keyword to help evaluating the derivative at one point, either for the respective trace, or through the condition reached with another.
Since this method doesn't seem to allow min, max, &amp; co, I would recommend you perform the same .dc analysis, but in .tran:

Set up your input as a ramp, e.g. PWL(0 0 1 2.5).
Assign {x} (or some other value) to your supply.
Step through the values of the supply with .step param x 0.5 2.5 0.5.

It's enough to plot V(out), only, which will show the correct response, but plotting against V(in) would mimic the .dc analysis. The advantage of this method is that, if you add a circuit that can perform the derivative of the output, you can use .meas to find the min/max/etc. The circuit is the group formed by G1 and L1, as described here. Its placement is similar, too: simply connect the noninverting input of G1 to V(out), and use the voltage across L1 for the derivative. I'd siggest a timestep 1000x smaller than the total simulation time (e.g. 1 ms for 1 s), or less, for sufficient accuracy (to avoid spurious spikes).
With that, if V(d) is the derivative, then simply use:
.meas d min v(d) from 10m

The from 10m ensures there are no random spikes in the beginning due to the derivative.