Removing that patch fixes the C++20 build problem, but a new FTBFS is now happening:
   dh_install -a
dh_install: warning: Cannot find (any matches for) "usr/lib/gambas3/gb.desktop.component" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-desktop missing files: usr/lib/gambas3/gb.desktop.component
dh_install: warning: Cannot find (any matches for) "usr/lib/gambas3/gb.desktop.gambas" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-desktop missing files: usr/lib/gambas3/gb.desktop.gambas
dh_install: warning: Cannot find (any matches for) "usr/share/gambas3/control/gb.desktop" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-desktop missing files: usr/share/gambas3/control/gb.desktop
dh_install: warning: Cannot find (any matches for) "usr/share/gambas3/control/gb.desktop/desktopwatcher.png" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-desktop missing files: usr/share/gambas3/control/gb.desktop/desktopwatcher.png
dh_install: warning: Cannot find (any matches for) "usr/share/gambas3/info/gb.desktop.info" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-desktop missing files: usr/share/gambas3/info/gb.desktop.info
dh_install: warning: Cannot find (any matches for) "usr/share/gambas3/info/gb.desktop.list" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-desktop missing files: usr/share/gambas3/info/gb.desktop.list
dh_install: warning: Cannot find (any matches for) "usr/lib/gambas3/gb.markdown.component" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-markdown missing files: usr/lib/gambas3/gb.markdown.component
dh_install: warning: Cannot find (any matches for) "usr/lib/gambas3/gb.markdown.gambas" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-markdown missing files: usr/lib/gambas3/gb.markdown.gambas
dh_install: warning: Cannot find (any matches for) "usr/share/gambas3/info/gb.markdown.info" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-markdown missing files: usr/share/gambas3/info/gb.markdown.info
dh_install: warning: Cannot find (any matches for) "usr/share/gambas3/info/gb.markdown.list" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-markdown missing files: usr/share/gambas3/info/gb.markdown.list
dh_install: warning: Cannot find (any matches for) "usr/lib/gambas3/gb.scanner.component" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-scanner missing files: usr/lib/gambas3/gb.scanner.component
dh_install: warning: Cannot find (any matches for) "usr/lib/gambas3/gb.scanner.gambas" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-scanner missing files: usr/lib/gambas3/gb.scanner.gambas
dh_install: warning: Cannot find (any matches for) "usr/share/gambas3/info/gb.scanner.info" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-scanner missing files: usr/share/gambas3/info/gb.scanner.info
dh_install: warning: Cannot find (any matches for) "usr/share/gambas3/info/gb.scanner.list" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-scanner missing files: usr/share/gambas3/info/gb.scanner.list
dh_install: warning: Cannot find (any matches for) "usr/lib/gambas3/gb.dbus.trayicon*" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-dbus-trayicon missing files: usr/lib/gambas3/gb.dbus.trayicon*
dh_install: warning: Cannot find (any matches for) "usr/share/gambas3/info/gb.dbus.trayicon*" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-dbus-trayicon missing files: usr/share/gambas3/info/gb.dbus.trayicon*
dh_install: warning: Cannot find (any matches for) "usr/lib/gambas3/gb.mysql*" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-mysql missing files: usr/lib/gambas3/gb.mysql*
dh_install: warning: Cannot find (any matches for) "usr/share/gambas3/info/gb.mysql*" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-mysql missing files: usr/share/gambas3/info/gb.mysql*
dh_install: warning: Cannot find (any matches for) "usr/lib/gambas3/gb.media.form*" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-media-form missing files: usr/lib/gambas3/gb.media.form*
dh_install: warning: Cannot find (any matches for) "usr/share/gambas3/control/gb.media.form/" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-media-form missing files: usr/share/gambas3/control/gb.media.form/
dh_install: warning: Cannot find (any matches for) "usr/share/gambas3/info/gb.media.form*" (tried in ., debian/tmp)

dh_install: warning: gambas3-gb-media-form missing files: usr/share/gambas3/info/gb.media.form*
dh_install: error: missing files, aborting


Full build log from the ppa: https://launchpadlibrarian.net/736856705/buildlog_ubuntu-oracular-armhf.gambas3_3.19.1-1ubuntu3~ppa1_BUILDING.txt.gz