Question

Does the ensemble of effective Lagrangians in the String theory landscape mostly include gauge theories?

Answer 1

In d=4d=4, there must be pretty much always a gauge group in the spectrum. It's not a theorem but there's strong evidence for it. Note that to find a gauge group, it's enough to find any spin-one light string excitation. A Yang-Mills symmetry must exist to render its negative-norm time-like polarization harmless - in fact, one may explicitly show by stringy methods that the gauge symmetry is there in any formalism that is Lorentz-covariant. The spin-1 fields arise either from the 10D gauge group e.g. in braneworlds or heterotic strings or from the RR fields or B-fields wrapped on cycles