So, you added coolant and had a little overflow? That by itself doesn't indicate that there's anything wrong at all. It's not at all uncommon to see a little overflow after you've topped the radiator off.
It's a little worrisome that your gauge never goes past the half way mark. Obviously it could just mean that your cooling system works really well, but it may also indicate that there's a problem with the gauge. If the radiator cap is in good working condition and all the connections are tight now you'll have to confirm that the gauge reads accurately.