Calculate DC-Gain from step response at high frequency
If your circuit is linear then DC gain is the gain at 0Hz. Nothing to add, nothing to remove. In your example the DC gain is 2. Nothing to add, nothing to remove - assuming your circuit is linear.

In a nonlinear circuit we have free hands te define the concept of the DC gain. We could declare for example how and from what frequency range we take the possible rectifying results into the account.

You have not given any declaration nor nonlinearity description. Thus the DC gain is 2 regardless of other frequencies. Nothing to add, nothing to remove.