  In this letter we investigate gauge invariant scalar fluctuations of the
metric in a non-perturbative formalism for a Higgs inflationary model recently
introduced in the framework of a geometrical scalar-tensor theory of gravity.
In this scenario the Higgs inflaton field has its origin in the Weyl scalar
field of the background geometry. We found a nearly scale invariance of the
power spectrum for linear scalar fluctuations of the metric. For certain
parameters of the model we obtain values for the scalar spectral index $n_s$
and the scalar to tensor ratio $r$ that fit well with the Planck 2018 results.
Besides we show that in this model the trans-planckian problem can be avoided.
