Their first season in this competition is more about gaining experience and understanding the intensity of competition that takes place in the UCL with against big teams, so indeed they can't do too much in the UCL when facing strong teams, like as Feyenoord, PSG and eindhoven who have experience and better performance in the UCL, however, like your said, the experience Girona have gained from UCL games this season will be invaluable for their future development, although there is no guarantee that they will be able to qualify for the UCL again if remember their inconsistent form in La Liga so far.
I feel that Girona did not do so well in the UCL this season of we look at how strong they were in the La Liga last season. I had expected them to be at the same level as Aston Villa who have been doing very well bith in the Champions league and the league as well but Girona is neither doing well in the champions league nor the La Liga. Assuming they were able to qualify for next round, they would have raised more funds to bring in some new faces to the club next season.