The IDL files are provided under the GPL as are all of Brutus. It does not matter whether you are using any of the actual servant implementation source code or not. All client applications that (in principle at least) are capable of using any Brutus object must use files that are derived from those IDL files. Those derived files, and the client application that are making use of them, are therefore, by virtue of section 2 of the GPL, covered by the GPL.

Therefore, your application, by using any of the IDL files, is covered by the GPL. This could naturally be what you want, but you must purchase a commercial license if your application is a software product that is not licensed under the terms of the GPL.