Like other breakout teams, Girona might find it very hard to cope with the intensity of the Champions league. It's not the Spanish league where they were familiar with all the teams they went up against. All the elite teams play in the Champions league and Girona will have much more on their plates. It's not just girona, but teams that arw prolific in their domestic league don't usually do well in UCL. See the likes or Salzburg or Newcastle as you have mentioned. I'd like to think that teams like that mainly consider their UCL qualification as an achievement and anything after that — getting past group phase, knockout stages, eyc as a bonus. Not to mentioned thru have sold some of their best players.
Several new teams will show their best strength in the Champions League, newcomers such as Girona, Leverkusen and Bologna will certainly be the scapegoat. But they can overcome that if they are able to provide a good run when facing other strong teams. Of course, it is very true that the Champions League is a fairly competitive league because there are many big teams that have stronger strengths, such as Real Madrid and Manchester City. This season, of course, the newcomers will be very stressed if they face a team full of experience in the Champions League like Madrid.
If big changes are made by management in the transfer market, of course there is great confidence that new arrivals can break many existing assumptions. So we'll just have to wait and see how they do in the Champions League this season.