Yes, the basic of GPL license is: if you use someone else’s GPL code (e.g. Sumatra’s code) you have to make your application also available under GPL license.
However, there is no such thing as “part of my application”.
You can’t say “those 100 lines of my application are under GPL but the rest isn’t”.
Either the whole application is under GPL license or none of it is.
By incorporating GPL licensed code you must license the whole application under GPL.
If you don’t want to do that, then you can’t use other peoples GPL code in your application.