Bitcoin is generally accepted in France however the government seems to be very willing to prosecute anybody who is in breach of reporting or other laws. Somewhat recent proof: http://www.reuters.com/article/2014/07/07/us-france-bitcoin-idUSKBN0FC19220140707
There is not much legislation on Bitcoin, however it is clear that you can not use new technology to break old laws. Accepting bitcoin as a form of payment for legal products/services is generally agreed to be the same as if you accepted USD or any other currency (for reporting requirements). You can pretty safely assume that accepting bitcoin in lieu of Euros is safe so long as Bitcoin is not illegal in your jurisdiction.
As for the legality of operating a casino/gambling site, there are much more considerations to be had. Of course operating such a casino/gambling site would have to be legal in your jurisdiction, also you should speak with a finance lawyer to determine if you need to register as a Money Transmitter or Money Services Business (since you may be taking money from one to give to another, as opposed to a store which simply takes money and might give a refund to the same client but never transfers money ON BEHALF of said client). You also need to consider which countries/regions you can serve and which you can not. For example, online gambling very illegal in the US, and simply having a US customer can be a breach of American law with hefty penalties. Most sites block US IPs for this reason.