FCGetDimensions[expr] is an auxiliary function that determines the dimensions in which 4-momenta and Dirac matrices of the given expression are defined. The result is returned as a list, e.g. {4}, {D} or {4,D,D-4} etc.
This is useful if one wants to be sure that all quantities inside a particular expression are purely 4-dimensional or purely D-dimensional.
FCGetDimensions[GA[i]]\{4\}
FCGetDimensions[GSD[p]]\{D\}
FCGetDimensions[FVE[q, \[Mu]] GS[p]]\{4,D-4\}