What is the value of a 1.5 gram twenty-five dollar gold coin?

Answer 1

Genuine US bullion gold pieces are sold by troy ounces rather than grams, so your "coin" is almost certainly something issued by a private company, and the $25 denomination is artificial.

Without knowing how much of that 1.5 gm weight is pure gold it's very difficult to say how much it's worth. It's not likely to be high purity (e.g. 99.9%) but taking that as an upper limit and using an average gold price of US$1600/troy oz (31.11 gm), its absolute maximum value would be around $75.

In reality it's probably less pure than that, and its value to a collector or investor would also depend on how much demand there is for products from the company that issued it.