I would like to get VCL:gtk3 instead of VCL:kde5 for LibreOffice 6.2.8.2-2.
How do you assume flatpack being related to either VCL:gtk3 or VCL;kde5. If your system (even if it is KDE) you can start it using VCL:gtk3 (as you know by my comment here). Just install all required GTK3 Libraries