It's not a G sound. The French [and German as well] R sound is made nearer to the throat rather than the lips like in English. It's like the Scottish word loch or if you put a lot of exasperation into the interjection UGH! [use the back of your mouth to make the gh sound], just a lot softer.
So, in conclusion, it should sound like oorchs.
Leurs = the "eu" is similar to the 'ea' sound in 'learn' and the 's' is silent (unless there's liaison, and then the 's' elides to a 'z' sound)
L'ours = similar to 'tour' and the 's' is not silent.
See here: http://translate.google.com/#fr/en/leurs%20l'ours - This link didn't really work. But just type in both words and press the speaker to hear them pronounced.
And here: http://www.forvo.com/word/l%27ours/#fr
"The bear is fat" is also accepted. I wonder how one would distinguish between 'fat' and 'big' in French? If I say he's a big man, that doesn't necessarily mean that he's fat, but that he's tall and perhaps muscular. To say that he is tall, one says "Il est grand". I would have guessed that "Il est grand" would have meant he is big, but apparently not.
Well in english many words can have many different meanings, like big can mean he is a big man as in he is a giant man or he is a big man could also mean he is a fat man. It also depends on context of big. So if there isnt any others words for it, then just have to deduce the meaning from the contexts which is something that is done in english too since its such a ambigous language. Hope this helps
Thanks PorquePuedo. But gros/grosse also means "fat" although it can also mean "big". Calling someone gras/grasse is an insult. Calling them "gros/grosse" is more polite. I asked this question on another discussion. Sitesurf and Jrikhal gave some really good responses .We also discussed "grand". http://www.duolingo.com/#comment/1030214
Bears plural in french is les ours. Bear singular in french is l'ours. Its is le ours a masculine noun, but because the e is a vowel next to another vowel it is elided this is called elision. So le ours becomes l'ours. The reason for this is to have language spoken more fluently, so remember two vowel sounds in a row stop the flow. Also I've heard there are exceptions but no need to worry about them now.