"number of datapoints, not the size of the inputs"

For brain imaging, 100k was 'the dimensionality of the input' (but also the number of datapoints). This could easily be O(10^6) though with higher res imaging.

I'd check out, for discussion:

https://www.prowler.io/blog/sparse-gps-approximate-the-poste...

For code:

https://github.com/GPflow/GPflow

Based on the people involved knowing what they are talking about, rather than on experience using that particular work.