By mentioning GLD, I presume therefore you are referring to the SPRD Gold Exchange Traded Fund that is intended to mirror the price of gold without you having to personally hold bullion, or even gold certificates.
While how much is a distinctly personal choice, there are seemingly (at least) three camps of people in the investment world.
First would be traditional bond/fixed income and equity people. Gold would play no direct role in their portfolio, other than perhaps holding gold company shares in some other vehicle, but they would not hold much gold directly.
Secondly, at the mid-range would be someone like yourself, that believes that is in and of itself a worthy investment and makes it a non-trivial, but not-overriding part of their portfolio. Your 5-10% range seems to fit in well here.
Lastly, and to my taste, over-the-top, are the gold-gold-gold investors, that seem to believe it is the panacea for all market woes. I always suspect that investment gurus that are pushing this, however, have large positions that they are trying to run up so they can unload.
Given all this, I am not aware of any general rule about gold, but anything less than 10% would seem like at least a not over-concentration in the one area. Once any one holding gets much beyond that, you should really examine why you believe that it should represent such a large part of your holdings.