That should be the offset hyperparameter of the linear kernel. The linear kernel is a bit different than other kernels, in that it is non-stationary, with the offset being the point in x through which all posterior functions pass through. https://www.cs.toronto.edu/~duvenaud/cookbook has a good overview of kernels and compositions of kernels. I find the linear kernel a bit strange to interpret, so if your case suggests a linearly decreasing trend you could also demean your data or express that relationship via a parametric linear mean function directly.
2 Likes