I tried using https://www.npmjs.org/package/gulp-changed, but couldn't get that to work in the timebox I allowed myself. Also, just passing the changed file to the typescript compiler would probably not work in our case (since we also need to pass in our type definitions in the same compiler pass), and recompiling a single file might lead to errors in a different file importing the changed file that you would miss with this setup...

The tsc executable does have a -watch flag itself for incremental compilation, which presumably takes into account these dependencies. So may be the solution is to use that outside of gulp (since gulp-tsc doesn't seem to be able to launch the compiler in watch mode).