Sounds like opinions vary quite a bit.
I think you are seeing such a large variety of answers because you have people who deal with construction contracts and/or work in the trades giving you advice in the same thread that Joe Blow homeowner is giving you advice. That's the beauty of the internet, I suppose. People who are not familiar with contract documents and the whole Supplier/GC/Owner/Subs equation are not going to see this the same way as someone who deals with them regularly. That is why I stated in my first reply that your best bet is to read the contract documents.
I am still not real clear on if you paid the supplier and the GC seperately or not. Let me run through a few scenarios.
If you bought the building supplies in one contract (a material contract), and have a completely seperate contract with the builder (building contract), then you own the left over supplies. This would also make you responsible if the builder had needed any additional supplies during construction unless the seperate contract you made with him makes him responsible for such supplies. It's all in how the contract is written.
If you paid the supplier for the materials (material contract) and they reccomended a GC that you then paid for through the supplier (building contract), then things get murky and a well written contract should address issues like this. It could go either way depending on the wording. You still have two seperate contracts, but they are getting paid through the same entity. In this case, I think you would still own the leftovers, barring the contract documents don't read otherwise.
If your contract with the supplier includes a GC to assemble the building, and you paid to have a completed building including the materials needed and the labor to complete the building (lump sum contract including both material and building contracts in one), then the material does not belong to you. It doesnt matter if you provided material money and progress payments or paid in one large lump, it doesn't belong to you because that is not the type of contract you signed. It belongs to either the supplier or the GC depending on their contract, but either way it is not yours unless their contract specifically states to give it to you.
These are just three possible scenarios, and there are several others that could have happened. Remember, anytime contracts are written, they are written in favor of the person writing it. Also, sometimes things are left vague on purpose. Without reading yours fully, it is hard to tell over the Internet what belongs to whom.
The GC may end up just giving you the extras, because it doesn't really sound like there is that much left over, relatively speaking. It may be easier to just cut his losses and let you have at it then fight for a grand or two of extra supplies.
Unfortunately, as the owner, you need to do your due diligence to not get screwed. Right or wrong, that is the way it is. I hate it just as much as the next guy, but it is what it is. Thank the cheats and the lawyers of this world, haha. Everyone involved in any contract needs to be careful and read it it front to back and back to front several times to ensure that it is as fair as it can be for all parties involved.
Sorry this is such a long post, I will be done after this one and just wait to see what ends up happening. It is hard to describe different types of contract scenarios over the Internet and have a meaningful discussion. Keep us updated and let us know how it turns out.