The landing gear are most definitely not visible to anyone on board, other than stowaways in the belly. But you can hear the gear being retracted and you can hear and feel the aircraft become more aerodynamic when the gear finish coming up. It’s strikingly apparent if you are tuned in... and when I fly I can’t help but pay attention to every little gnat’s eyelash that happens to make a peep. I should mention.... I HATE FLYING!!!
Wait... upon consideration... I must say that I can’t confirm if the rear gear are visible from the forward or aft passenger windows. My apologies for sounding so sure when I’m not.
Based on this video, they very well may be visible from the forward or aft seats.
https://www.youtube.com/watch?v=lON1ciKY0MU