Some problems

That would mean that each bitcoin/goldcoin would have a fixed value and cannot rise in value as more people adopt it.
Ofcourse it could raise in value simply by adding more gold at a constant rate.

It would also mean that the value would be decided by the speculation in gold instead of reflecting how useful, adopted and good the bitcoin services are. Removing the very important incentive to create good services.

And question, how would you decide how much gold each coin should be worth?

But ofcourse, you are free to create such a coin.

You just need one of the worlds best securities.