Papers exist on how to convert between the Gaia G, RP and BP measurements to other wide-band systems such as Johnson-Cousins. For example, see Appendix C and its two tables from https://arxiv.org/pdf/2012.01916.pdf for a detailed account.
I have a large number of unfiltered images with stars between 8th and 22nd magnitude, taken with two different CCD cameras. Perhaps I should measure them and see how well they correlated with DR3 values.