It would depend on if it is your first visit, or has been there before.

In China, you do not bring one gift, according to supperstition, gifts must be in pairs at least (or even numbers).  You may try finding out if she likes chocolates, or a special brand of tea, or a basket of fruits, or ask GF what her mama likes and get two of the suggestions. 
Remember, it is not the price of the gift, but the thought behind it.  And hand it to her using your both hands.

Can't go wrong with Western chocolates. Or if she's really into cooking then bring American ginseng. 

One gift that I think your mother in law or mother of your girlfriend would enjoy would be something that she will use more often. You could get some cooking appliances such as pots and pans or even cutlery. If your mother in law or your girlfriends mom does not cook much you couldn't go wrong with some lovely flowers.

Depends on how big of a deal this is. I met my wife's parents just before the wedding, so I plied them with shit my parents brought from home, smoked salmon, ice wine, scotch...etc. 

get her a foreign "stripper" and a hotel room. you will instantly become her favorite son in law

Hong bao, stuffed to the gills! Always appreciated and always lets them know you are serious.

Dead Mao's (RMB) are accepted anywhere, and can  easily be turned into the families favorite gift, or more likely than not, saved for a rainy day, or invested in the soon to crash property or stock markets.

Also in China, unlike some other country's, the husband (or his family) pays the dowry to the family of the bride, especially amoung older people and villagers, because you are taking the daughter from the family.

It will make you a dowry contender!

Oh BTW, you marry a Chinese lady, you are reponsible for the care of the mama and baba (extended family) for life!

I accepted that. I bought them a HOUSE! Can you? You need to think about this!

Aw, heck, just marry the girl already!