Just thought I leave this here in case anyone is looking. Was using Ubuntu 20.04 LTS with WSL2 to build an addon for LibreELEC 9.2, I noticed that building GTK3 would fail at building gdm as Ubuntu 20.04 no longer has libffi6 but only libffi7. For some reason it still tries to look in /usr/lib/x86_64-linux-gnu, where libffi.so.6 doesn't exist. It is there in toolchain but for some reason it doesn't use that and still tries to look in /usr/lib/x86_64-linux-gnu.
Copied libffi.so.6.0.4 from LibreELEC.tv/build.LibreELEC-Generic.x86_64-9.2-devel/toolchain/lib/libffi.so.6.0.4 over to /usr/lib/x86_64-linux-gnu, and created an symbolic link for libffi.so.6 solved the issue.
The issue doesn't occur with master.