Flight information to GRX airport:

We can fly to Granada airport with a transfer at Barcelona (BCN), Madrid (MAD), Palma de Mallorca (PMI), Paris-Orly (ORY), or London-Gatwick (LGW) airports. However, the most convenient option for all cities (Gdańsk, Katowice, Kraków, Poznań, Szczecin, Warsaw, and Wrocław) would be to connect to Malaga (usually direct) and take the Alsa bus directly from the airport to the bus station in Granada. There are about 13 trips per day, and the journey takes 2 hours and 15 minutes. Considering that it takes about 20 minutes from the GRX airport to the bus station in Granada, the time difference is less than 2 hours. The fact that Alsa buses transport bicycles (preferably packed as in an airplane) for a small fee (around 5€) is also a reason to choose the journey through Malaga airport (AGP).
We can also take a train (3 trips per day) from Malaga airport to Granada much faster - first on the C1 line to Málaga María Zambrano station, then change to a train to Granada. The entire journey takes from 1 hour and 35 minutes to 3 hours.
If we do decide to travel to GRX airport, we can find the bus timetable for line 0245 (GRX - Granada) here.