jenpet - your gite looks wonderful! (I have saved it as a favourite - it's a part of Brittany that we love)
overthemill - no direct experience, but an acquaintance of mine owns a house with four cottages that she rents out in Wales. (Not quite the same!)
I know from her experience that it's hard work and doesn't generate a huge amount of income.
As well as knowing how many weeks of the year you are likely to be occupied, you also have to take into account the time each week you will have to spend cleaning the gites for the next visitors. And, sadly, some people do leave them in a dreadful state. If you're not going to do the cleaning, then you will have to pay someone else to, which will eat into your profits.
It's also very tying - the holiday season is her busiest time of the year, so it's hard for her to take a break with her own family.
Getting the right agent and/or developing your own website are crucial for promoting your business. If you go through an agent, there are all kinds of things that they will tell you that you have to have/do - for heath and safety, to reach a certain star standard and/or to cater for those who are less abled and for families with children.
But I'm sure you know all this, and that there are many positives as well, so good luck!