If you go to Document > C/C++ Options..., there is an options page for Compiler Properties. Starting SlickEdit with your gcc 10.2 on your path, if you click on "Add..." to add a new configuration, if SlickEdit can detect your installation (I think this does depend on the bin/ directory having a gcc or g++), then it should be there for you to add. SlickEdit does this auto-detection once, so that's why you would have to hit add to find additional compiler installations if you need to add one after the fact. If it detects something new, you will get a dialog box "Add Compilers" with a list of compiler names. If there is nothing new, then it will prompt you for the name of a compiler to add manually.
The compiler configurations are stored in your configuration directory (compilers.xml).
The setting in the XML project files (*.vpj) for compiler configuration is "CompilerConfigName", which is per-configuration. There is some logic built-in if you set the project compiler configuration setting to "Default Compiler" or "Latest Version" to select the default compiler configuration (as set up on the C/C++ Compiler Properties dialog) or the configuration that SlickEdit is able to parse out with the highest version number (that parsing is somewhat specific to how SlickEdit detects and names compiler configurations).
You could edit all your project files and set "CompilerConfigName" to your 10.2 configuration name, or the "Default Compiler", or to "Latest Version" to use that configuration consistently across all projects.