Gain at critical frequency for Butterworth and Chebishev filters
If I understood your question correctly, Butterworth filter is maximally flat (no ripples) in the passband frequencies, and at the cutoff frequency, the gain is down by 3dB (3.01 to be exact). But disadvantage is slow roll off from passband. See https://en.wikipedia.org/wiki/Butterworth_filter especially the figure showing magnitude roll off.

For Chebyshev filter type 1 filter, the roll off is more sharp but result is ripples in pass band. You can control this ripple, which will in turn vary the Gp that you mentioned. See https://en.wikipedia.org/wiki/Chebyshev_filter. If you specify $\delta$ as the ripple factor in dB scale (like 1dB), $\epsilon = \sqrt{10^{\delta /10}-1}$ in linear scale. The gain Gp = $\frac{1}{\sqrt{1 + \epsilon^2}}$ in linear scale. The sharpness of your passband and passband ripple specification will affect $\epsilon$ which will inturn affect Gp.