How to make apt ignore unfulfilled dependencies of installed package?

You can’t make apt ignore dependencies, but you can create a fake gstreamer0.10-plugins-good package which will satisfy the missing dependency. The simplest way to do this is using equivs:

  1. install equivs

    sudo apt install equivs
  2. generate a template control file

    equivs-control gstreamer0.10-plugins-good.control
  3. fix the package name

    sed -i 's/<package name; defaults to equivs-dummy>/gstreamer0.10-plugins-good/g' gstreamer0.10-plugins-good.control
  4. build the package

    equivs-build gstreamer0.10-plugins-good.control
  5. install it

    sudo dpkg -i gstreamer0.10-plugins-good_1.0_all.deb

That should satisfy the opera package’s dependency.

You can also remove the gstreamer0.10-plugins-good dependency of the opera package by editing /var/lib/dpkg/status.

Just open it with a text editor, search for the line Package: opera and under it in the Depends: line remove the offending gstreamer0.10-plugins-goodpackage.

After that apt works again.