Lincense question for commercial app

[Karl Berry]

    This means that GPL software _must_not_ be used in a closed source
    software.

It depends on what "used" means. For example, if the app is merely
executing a pdftex binary, then providing the sources that that built
that binary is sufficient.
https://www.gnu.org/licenses/gpl-faq.html#MereAggregation

Zdenek, in your case, wanting to link a GPL'd library, indeed,
I agree with your analysis. You are making one executable
-> not mere aggregation 
-> all sources must be made available. --best, karl.