When you install SuSE, it installs a database of packages available on the installation media, and which of those packages you chose to install.
Anything else, from anywhere else, wil not be in that database.
Solution: When you go into YaST, instead of clicking on Install/Remove software, click on Change Source of Packages (or some such name). You may need to do some www.google.com/linux
research first. Find a repository of SuSE packages which contains the package you want. Then edit the Source in YaST to include the URL for that repository.
Then, when you go into YaST Install/Remove software, the contents of that repository should also be displayed (assuming you are on-line at the time).
For apps that would ordinarily be on the installation media, such as gcc and gtk, instead of Search, from the same drop-down dialogue, select Package Groups. Scroll down to the last line and click on ZZZall, to get the full list of packages available on the installation media.
They are listed in alphabetical order. Scroll down to the g's. If gcc and gtk are not installed, click once on the checkbox, click Apply, supply the necessary cd/dvd, and be done with it.