What is the value of 925 Italy gold necklace?

If the necklace is stamped with a “925” hallmark, it is likely that it is made out of sterling silver, and has gold plating. Sterling silver is an alloy, which means that it is a mixture of metals. “925” refers to the silver content in the alloy, i.e., that it is made from 92.5% plata.
Silver has some value, but not nearly as much as gold. The gold plating on the necklace will not be very thick, and the necklace will not be as valuable as if it were made from pure gold or a gold alloy.
If you are looking to sell the necklace, I recommend taking it to a local reputable jeweler and confirming with them that the necklace is indeed gold plated silver. A 925 stamp means that the base metal is very likely silver, but it is better to be certain before you attempt to sell the piece.