After a small bit of research, it looks like most of the information related to the LCL GTK3 can be found on this page. I am unsure how up to date the issues are as the README.txt file was last updated 8 years ago.
https://gitlab.com/freepascal.org/lazarus/lazarus/-/tree/main/lcl/interfaces/gtk3 >
-- _______________________________________________ lazarus mailing list lazarus@lists.lazarus-ide.org https://lists.lazarus-ide.org/listinfo/lazarus