Where is gcc-doc package?

Perhaps adding contrib your sources.list helps, if you decide to go with by yourself adjusted version of (remove or block this line afterwards):

It is very late for me but now try only:

@Usr1, please do not execute or even think to execute sudo apt upgrade, not at all and add ## in front of deb https://ftp.debian.org/debian/ ... line or just delete it, after above successful (hopefully)!