Hey! I was wondering if "najbaroj" was neuter or masculine? Since "najbarino" means female neighbour it'd follow that "najbaro" would be masculine while "genajbaroj" would be gender neutral? This however doesn't seem to be the case judging from the lessons and a quick look at the dictionary.
The majority of nouns are neuter. Most masculine nouns are for family members, nobility, some religious members, and some animals. Feminine nouns are even fewer. Nouns designating people in general are always neuter.
You can use najbarino if you want to specifically emphasize you're talking about a female neighbour, but the root is neuter.
Bertilo has a page about that : https://bertilow.com/pmeg/gramatiko/o-vortoj/seksa_signifo.html?s=x/%3Fs=x
In addition to what Louis rightly said, "genajbaroj" is pretty much the opposite of gender neutral: It means specifically a mixed group of male and female neighbours, not accounting for the possibility of someone gender non-conforming (please give Dr Zamenhof a break; in his time, I bet even gender non-conforming persons themselves didn't have the mental space or tools to themselves think their own identity).