Rangers game against Panathinaikos will definitely be difficult to predict right now because both teams finished second in their main leagues last season, and we don't know how strong either team will be right now. Rangers will host the first leg at Ibrox Stadium, and the bookies have Rangers @1.86 odds and Panathinaikos @4.02 odds. Rangers seems to be the favorite team because they will host the match. While I believe some bookies would prefer the Rangers odds because they will be playing at home.
In addition, both clubs failed to win some of their last friendly games because they are afraid of risking their top players for the next Champions League qualification. The match will be played next Wednesday, so I'm not making any predictions right now; but if Rangers win, I won't be surprised because they will be hosting the game and may have a home advantage.
I doubt it is just because the game is at their home, when you have odds like that, even if it was in Greece, rangers would have been favourites, obviously the odds would be closer, but it would still be rangers that are favourites. However, when it is this early into the season, I would say there aren't any clear favourites, considering how most of the teams are just getting started on the training, and mainly not even training their best players.
I have seen some friendlies and they were playing either the young players, or their bench players, with only a few starters playing. Which means, neither team is ready, nobody can be ready this early into the season, how can you be ready in July, so that could give Pana some advantage, catching Rangers off guard and not ready would be their only chance.