  In this paper, we study gamma-positivity of descent-type polynomials by
introducing the change of context-free grammars method. We first present
grammatical proofs of the gamma-positivity of the Eulerian polynomials, type B
Eulerian polynomials, derangement polynomials, Narayana polynomials and type B
Narayana polynomials. We then provide partial gamma-positive expansions for
several multivariate polynomials associated to Stirling permutations,
Legendre-Stirling permutations, Jacobi-Stirling permutations and type B
derangements, and the recurrences for the partial gamma-coefficients of these
expansions are also obtained. Moreover, we define variants of the Foata-Strehl
group action which are used to give combinatorial interpretations for the
coefficients of most of these partial gamma-positive expansions.
