Interesting stuff. The GPU calculation documentation is here under the InputOutput reference:
https://bigladdersoftware.com/epx/docs/9-3/input-output-reference/group-simulation-parameters.html#inputs-10-019
It looks like the whole ShadowCalculation object got an overhaul in E+ 9.3, which is likely to break some things in honeybee. It also seems like this GPU method is slower but it gives users a means to run detailed indoor solar distribution without needing to worry about concave zones (something I know a lot of people on this forum will be happy about).
The best I can offer until we upgrade honeybee is to write out an IDF, upgrade it to 9.3 by opening it in the IDFEditor, manually change the ShadowCalculation object, save it, and use the Rerun IDF component to run the model through E+ and import results to Grasshopper.
Once we upgrade honeybee, you’ll be able to do this all in a more streamlined manner like by using a OpenStudio Measure.