  We introduce a string-based parametrization for nucleon quark and gluon
generalized parton distributions (GPDs) effective across all skewness values.
The conformal moments of the GPDs are expressed as sums of the spin-j nucleon
A-form factor, and the skewness-dependent spin-j nucleon D-form factor. This
representation, which fulfills the polynomiality condition and does not rely on
model-specific assumptions, is derived from t-channel string exchanges in AdS
spaces. The spin-j nucleon D-form factor is closely related to the the spin-j
nucleon A-form factor. We use the Mellin moments from empirical parton
distributions to model the spin-j nucleon A-form factors. Using only five Regge
slope parameters, fixed from the electromagnetic and gravitational form
factors, our string-based parametrization generates accurate singlet,
non-singlet, isovector, and flavor-separated nucleon quark GPDs, along with
symmetric nucleon gluon GPDs from their Mellin-Barnes integral representations.
Our isovector nucleon quark GPD is in agreement with existing lattice data. Our
string-based parametrization should enhance the empirical extraction and global
analysis of nucleon GPDs in exclusive processes, bypassing the de-convolution
problem.
