How did you achieve this?
One of the biggest benefits of G-trees in my mind, is their ease of implementation. Additionally, we did a lot of work to explore their statistical properties, which doesn't exist for prolly trees (though in hindsight, we have done this, so should probably write it up formally).
If you want to break that up with dummy elements, you now have the problem of choosing those dummies in a history-independent manner efficiently.
But I think their recursive construction with G-trees of G-trees of ... might work if nodes with too many elements are stored as G-trees with a different, independent rank function (e.g. using a hash function with different salt). Producing many nodes with the same ranks should then get exponentially harder as the number of independent rank functions increases.
As a small comment, this seems closely related to another recent paper: History-Independent Dynamic Partitioning: Operation-Order Privacy in Ordered Data Structures (PODS 2024, Best Paper).
I'm not sure how they compare, since neither paper seems to know about the other. And I'm also not sure which paper came first, since the geometric search paper does not seem to post a publication date.