Girona's quality may only be good for La Liga and to measure how well they can manage their busy competition schedule they will have to try to get a place in the champions league next season. I agree that every team must have reserve players who are as good as the main players and that is what Real Madrid and Barcelona currently have, so however, Girona, with the depth of their current squad, I don't think can match these two teams in the title race. Being able to finish in the top 4 is a very realistic thing for them.
Girona's team has the potential to compete in leagues other than Laliga. They are simply a team that has transformed from a small to an average team; their performance this season reveals a lot about the team's transformation, and with the number of matches they have won in all competitions, I believe they can represent Laliga even in the Champions League; experience will be their limit, but they will do well because they have quality.
Squad depth will only have an impact on the team because not only injuries but also fatigue will have an impact on their performance, which is why Real Madrid and Barcelona will finish ahead of them in the Laliga title race this season, but there is assurance that they will make it to the Champions League next season if they maintain their consistency.
It is still a big step to go from the top of a domestic league like La Liga to the next level in the Champions League. And we have a good example for that, which is Newcastle United. They are playing in an even stronger domestic league and they did play very well last season in the Premier League and yet it doesn't mean that it is also time for them to be dominant in Europe. Can Girona take the next step? Yes I think so and considering who is backing Girona, it is possible that investments will follow if they qualify for the Champions League. But I would not expect an outstanding first Champions League season for them. It is more like acclimatization to the new environment and then get going better and better over time in the coming years.