Ali Pesaranghader

@mahdiabdollahpour try ```python from skopt.learning.gaussian_process.kernels import RBF ``` instead of ```python from sklearn.gaussian_process.kernels import RBF ``` It seems that Scikit-Optimize has a wrapper that implements `gradient_x`: https://github.com/scikit-optimize/scikit-optimize/blob/master/skopt/learning/gaussian_process/kernels.py#L68

You may want to change the default Kernel from Matern to RBF (or any other) by editing the `utils` file: https://github.com/scikit-optimize/scikit-optimize/blob/master/skopt/utils.py#L380 ```python if is_cat: other_kernel = HammingKernel(length_scale=np.ones(n_dims)) else: # other_kernel...