If you want to know how consistent a team will be throughout the season, just look at the depth and quality of their bench. It is clear that Real and Barcelona are superior to Girona in this indicator very significantly, so everything is obvious. While Barcelona had problems with injuries, their results were worse, now everything is getting better.
These factors are important but even with best players, a club can have bad performances and they even can have it stably during a long season. Last season, Real Madrid had a better squad than Barcelona but they lost La Liga to Barcelona. This season we are witnessing Girona with very limited player resource, worse than Real Madrid and Barcelona, are showing very impressively and challenge a first and second positions.
Years ago, in Premier League there was Leicester City suddenly won the league title against many big clubs. They were similar to Girona in this season but will Girona have a happy ending and be able to lift the trophy, I am not sure.
But Girona now also does not have big problems with injured players, if injuries occur, they are often not significant, but I believe that it is still more difficult for Girona to compete with the giants of La Liga.
They will hardly repeat it next season but in this season, they have done great jobs and they don't need to win La Liga to make it as a most successful season in their club history.