What exactly do you want to oversample?
Gqrx already oversamples automatically. For example, for an rtlsdr you
can choose 1.5 Msps. This is way more tat needed for e.g. FM voice so
it is downsampled to 96 ksps for all arrow band modes and demodulated
at that rate. If you want to change that you have to change the source
code.
http://gqrx.dk/doc/gqrx-design
Alex