The answer to that is probably "the smallest gold coin you can find"
As has been said really gold coins are expensive (especially with the gold price as it is currently) but there are still a few coins possible.
I picked up some gold coin from a Pawnbrokers for a very slight premium above the gold melting price (these were taken from jewellery ready to be melted...and the coins would have been melted too)
They were probably EF condition, sovereigns and half sovereigns with some age to them... (these weigh 8grams and 4 grams) at the time the gold price was really low but now you would obviously get a lot less for your money.
So it is possible you can get some gold coins for around $120 but they wont be rare or in mint condition... I agree you should stick to non-gold. I have a Statue of Liberty gold coin from the states (gold rate has pushed that up lots for me) I think it is very attractive coin but realistically had I spent that same money on silver I would have many more pieces and it would be more interesting a collection. SO I agree with these guys...gold isn't such a good idea...