The Offset option of balredOptions sets the boundary between poles that balred can discard, and poles that balred must preserve (treat as unstable). Setting Offset to 0.0005 preserves the two stable poles of gasf35unst that are closest to the imaginary axis.
Providing balred an array of target approximation orders [10 15] causes balred to return an array of approximated models. The array gasf_arr is contains two models — a 10th-order and a 15th-order approximation of gasf35unst. In both approximations, balred does not discard the two unstable poles or the two nearly-unstable poles.
Compare the reduced-order approximations to the original model.