The owners of Girona are City Football Group and Girona Football Group, while City Football Group is one of the companies developing in the football industry and has the largest stake in this industry.
This company is owned and founded by Sheikh Mansour but City Football Group only has 44.3% shares or you could say only half because half of the shares are owned by Girona Football Group itself.
In contrast to Manchester City, it is clear that 100% of the shares are held by the City Football Group and this means that all control and ownership rights of Manchester City are under the hands of Sheikh Mansour.

Indeed, UEFA regulations do not allow two clubs that have the same owner to take part in European competitions such as the UCL, but because Girona is not completely owned by the City Football Group, there could be considerations made by UEFA.
It just that I don't really know what it will be like next season because these two teams are really quite strong this season and have big chance of getting UCL tickets.

From this thread I remembered case that occurred in the past where UEFA allowed Leipzig and Salzburg to compete in the UCL and there was debate over the same ownership, namely Red Bull, an energy drink company.