Using the wrong gender...
How bad of a faux pas is it to use the wrong gender for a word? For example, if I was in France and I said "la pain" instead of "le pain," would that be that big of a deal? French people must get them wrong sometimes too, right?
It's probably the same like with the genders in German (my native language). Everybody will understand you if you use the wrong gender for a word, but everybody will instantly know that you aren't a native just because of that error. We don't make that error with common words, so it's a clear sign no matter how good your pronunciation is. But it's normally no big deal at all.
I think there are probably words where making a gender mistake could be a bigger issue, but I would imagine that most French people would understand what you are trying to say. Also, from what I've been told, only French children make mistakes with their gender. I imagine it's comparable with the way English speakers rarely make mistakes regarding singular nouns having plural verbs (ie 'The dog eats." vs "The dogs eat.") which seems to confuse some English-as-a-second-language speakers.
Sorry to upset you, but French words that vary in meaning depending on gender certainly do exist! For example, un livre is a book, but une livre is a pound (imperial weight or British currency). In the scheme of things, there aren't a great many of these sorts of words, but they are there, lurking about...
A French word which would have two senses according to its gender doesn't exist to my knowledge.
A French. ;)