DiracGammaCombine[exp] is (nearly) the inverse operation to DiracGammaExpand.
Overview, DiracGamma, DiracGammaExpand, DiracSimplify, DiracTrick.
GS[p] + GS[q]
ex = DiracGammaCombine[%]\bar{\gamma }\cdot \overline{p}+\bar{\gamma }\cdot \overline{q}
\bar{\gamma }\cdot \left(\overline{p}+\overline{q}\right)
ex // StandardForm
(*DiracGamma[Momentum[p + q]]*)2 GSD[p] - 3 GSD[q]
ex = DiracGammaCombine[%]2 \gamma \cdot p-3 \gamma \cdot q
\gamma \cdot (2 p-3 q)
ex // StandardForm
(*DiracGamma[Momentum[2 p - 3 q, D], D]*)DiracGammaCombine[2 GSD[p] - 3 GSD[q]]
DiracGammaExpand[%]\gamma \cdot (2 p-3 q)
2 \gamma \cdot p-3 \gamma \cdot q