We reported a type-I difference-frequency generator (DFG), based on erbium doped GaSe (Er:GaSe) crystals as a coherent infrared source tunable from 2.4 μm to 28 μm. The two mixing beams used for the DFG are a tunable near infrared output (1.1-1.8 μm) from an optical parametric amplifier (OPA) and the fundamental beam of a picosecond Nd:YAG laser at 1.064 μm. The system can produce a maximum output pulse energy of 5 μJ at wavelength of 3.5 μm, corresponding to a photon conversion efficiency of 8% at a pump intensity of 1.7 GW/cm2. The nonlinear coefficient (deff) of 0.5 atom% erbium doped GaSe crystal was found to be 55.3 pm/V or 24% higher than that of a pure GaSe crystal. The improvement of deff is attributed to the substitutive and interstitial doping of Er ion in GaSe unit cell. The optical properties of GaSe influenced by the erbium doping are also presented.