How do I tell BMK to build a GUI app?

Found it. -t gui :)

Had to search the huge MAXIde.bmx to find it though :)

So a full release build for a gui app would be:

makeapp -a -r -v -t gui MyApp.bmx

Omitting -t gui will build a console app, and -a triggers a complete rebuild. -v is the verbose option and -r is for release build.