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We studied the gamma-ray emission mechanisms from pulsars with period, $P$, between $4.6 \times 10^{-2} B_{12}^{2/5}$ s and $0.17 B_{12}^{5/12} \sin^{1/6} \theta \alpha^{-5/4}$ s in terms of outermagnetospheric gap model. We found that the spectra of all known $\gamma$-ray pulsars can be fitted by two free parameters, namely, $\alpha r_L$, the mean distance to the outergap, and $\sin \theta$, the mean pitch angle of the secondary $e^{\pm}$ pairs. Gamma-rays from those pulsars with $P < 0.17 B_{12}^{5/12} \sin ^{1/6} \theta \alpha^{-5/4}$ s are mainly emitted by secondary $e^{\pm}$ pairs, which are created beyond the outergap, via synchrotron radiation and the gamma-ray emission efficiency is $\sim 10^{-2}$. For pulsars with period approaching $\sim 0.17 B_{12}^{5/12} \sin ^{1/6} \theta \alpha^{-5/4}$ s, their gamma-ray emission efficiency is approaching unity. We used our model to fit the observed spectra of $\gamma$-ray pulsars (Vela, PSR1706-44, PSR1055-52, PSR1509-58, Geminga). All the best fit curves satisfy the constraints of $\alpha$ and $\sin\theta$. The pulse separation and relative intensity of pulses are function of $\alpha$. In our model, the first three strongest theoretical $\gamma$-ray sources have been detected. PSR1951+32 is predicted to be the fourth strongest $\gamma$-ray pulsar (Cheng and Ding, 1994, ApJ , {\bf 432}, 724) which is confirmed by the recent GRO result.
