I would start off by changing the equations to something like
let
X = (Pratio^(gm1/-A.gam) * (1+(gm1/2)*Minf^2) - 1) * (2/gm1);
in
if X >= 0
Mloc == sqrt(X);
else
Mloc == {0,'appropriate_unit'};
end
end
In reality, most Simscape language blocks will have some sort of "smoothing" to prevent the sharp discontinuity -- usually a linear ramp down to zero over some threshold range (see, e.g., the Translational/Rotational friction blocks). You can try that further step if you run into problems, though.
- Sebastian
