How to apply the gain factor G to SOS coefficients when designing a IIR filter on sptool (MATLAB)?
The easiest way would be
y = prod(G)*sosfilt(SOS,x);

An alternative would be put to the cumulative gain into the first section.
sosScaled = SOS;
sosScaled(1,1:3) = sosScaled(1,1:3)*prod(G);
y = sosfilt(sosScaled,x);