Yes - when you select Gnome-mplayer in YaST, it should give the list of dependencies. Simply uncheck those you don’t want. You will probably get an error message but it should go ahead.
I strongly suggest that you never ignore dependency matters. The word ‘dependency’ says it all. Chances are you will be left with an instable system. Yes, the app may run now, but some future action may require the deps, bringing crashes etc etc.
You’re new to openSUSE, please stick to installing the available stuff first. If you need packages go Software.openSUSE.org, it may very well provide what you need.
Yes, read “man zypper”, although I would advise against it for the same reason I would not use apt-get source.
Get the src.rpm and unpack/install it as normal user, you can do all the steps needed for modifying/building a package as normal user, so why do it as root?
as normal user, now you can build your own packages inside your $HOME/myrpms without needing root privileges except the only action they are really needed for, installing/removing packages.
You can even add this directory as a local repo (plaindir) and use it with zypper/YaST.