magnetic combinations of unpolarized gluon GPDs and for the following combinations of polarized gluon GPDs: One may come to the same conclusion employing the general method for determining the invariant amplitudes of a binary scattering process suggested in [34, 35] (see also [25]). Using this method it is also straightforward to check that the electric combination H g ( E ) and pseudoscalar combination ˜ H g ( PS ) are to be expanded in the Legendre polynomials of cos θ$_{t}$ P$_{l}$ (cos θ$_{t}$ ) while the magnetic combination H g ( M ) and ˜ H g are to be expanded in the derivatives of the Legendre polynomials P ′ $_{l}$(cos θ$_{t}$ ). Note that to the leading order of 1 / Q expansion the t -channel scattering angle 3 is expressed through the kinematical variables as Taking into account all these considerations one can write down the following partial wave expansion for electric and magnetic combinations of unpolarized gluon GPDs in the nucleon $^{4}$: H g ( E $^{)}$( x, ξ, t ) = ∞ ∑ n =1 odd n +1 ∑ l =0 even B g ( E ) nl ( t ) θ ( 1 − x 2 ξ 2 )( 1 − x 2 ξ 2 ) 2 C$^{5}$ 2 n − 1 ( x ξ ) ξP$_{l}$ ( 1 ξ ) ; (18) ˜ H $^{g}$( x, ξ, t ) ; ˜ H g ( PS $^{)}$( x, ξ, t ) = ˜ H $^{g}$( x, ξ, t ) + τ ˜ E $^{g}$( x, ξ, t ) . (16) H g ( E $^{)}$( x, ξ, t ) = H $^{g}$( x, ξ, t ) + τE $^{g}$( x, ξ, t ) ; ( τ ≡ t 4 m 2 N ) H g ( M $^{)}$( x, ξ, t ) = H $^{g}$( x, ξ, t ) + E $^{g}$( x, ξ, t ) . (15) cos θ$_{t}$ = 1 ξ √ 1 − 4 m 2 N t + O ( 1 Q 2 ) . (17) For the electric combination of gluon GPDs H g ( E ) J PC = 0 $^{++}$, 2 $^{++}$, ... intermedi- ate meson states ( e.g. f$_{0}$ , f$_{2}$ ) contribute, while for the magnetic combination H g ( M ) the contributions of J PC = 2 $^{++}$, 4 $^{++}$, ... intermediate meson states are relevant. For ˜ H g J PC = 1 $^{++}$, 3 $^{++}$, ... intermediate meson states contribute and, finally, for ˜ H g ( PS ) J PC = 0 − $^{+}$, 2 − $^{+}$, ... meson states contribute. 8 3 θ t is defined as the scattering angle in the center of mass frame of the t -channel of the hard exclusive electroproduction reaction ( e.g. γγ ∗ → N ¯ N for the DVCS). 4 Note, that contrary to the case of quark GPDs in nucleon defined in [22] we do not introduce the factor “2” in the partial wave expansions for gluon GPDs.