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## I. INTRODUCTION 

Generalized parton distributions (GPDs) [1, 2] are considered to be a promising tool to study the partonic structure of hadrons (see Refs. [3–6] for recent reviews). Hard exclu- sive reactions, which can be described in the theoretical framework of GPDs, provide us opportunity to access the information on GPDs experimentally. 

The importance of GPDs was widely realized in connection with the possibility to study the total angular momentum of partons in the nucleon. This allows to address the funda- mental question how the total nucleon spin is made up from the contributions due to quarks and gluons. The famous Ji sum rule [2] relates the appropriate Mellin moments of quark and gluon GPDs 1 H $^{q,g}$( x, ξ, t ) and E $^{q,g}$( x, ξ, t ) to the fractions J q,g of the total angular mo- mentum ( i.e. the sum of parton spin and orbital angular momentum) carried by quarks of the flavor q and gluons respectively: 

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Constraining J g from the experiment would add an important lacking “piece” to the tricky “nucleon spin puzzle”. This require the knowledge of gluon GPDs H $^{g}$, E g as func- tions of x for fixed values of ξ and t . These GPDs can be most preferably studied in the hard exclusive electroproduction of J P C = 1 −− mesons ( ρ $^{0}$, ω, φ ). For these meson electro- production channels H g and E g make contributions at leading order in 1 / Q ( Q 2 refers to the initial photon virtuality) and in α$_{s}$ . Hard exclusive electroproduction of mesons and particulary the exclusive electroproduction of ρ 0 is now in focus of intensive experimental investigations (see e.g. [7]). The important set of data have been already published [8]. Future experiments will provide even more precise data over a broader phase space. Thus, further theoretical development of GPD formalism that would allow the interpretation of this new experimental information is highly demanded. 

Unfortunately, the problem of GPDs extraction from the data is complicated by the fact that GPDs depend on several variables (longitudinal momentum fraction of partons x , skewness parameter ξ , momentum transfer squared t and the factorization scale). More- 

