I'd give it a F, fine grade. I think reverse is Sol holding globe. Size looks like a sesterius. Ancient coins are often graded poor, fair, good, very good, fine, extremely fine, mint state etc.
For value you could use the sold search line on
ebay and sold prices on ancient coin auctions. As with modern coins the value of ancients depends on condition, market demand and rarity. In
ebay search line, type in Gordian III, look at the prices and condition of coins sold for comparisons.
One popular place to buy ancients is Vcoins.com. You can use search line on Vcoins, CNGcoins, acsearch.info and other sites to find more info for the ancient coins you posted including their reference numbers. On Vcoins I saw similar examples as your Gordian III ranging from low grade for $38 to about $200 in better condition.
Of course there are fake ancient coins so buy from dealers who know about them or offer refunds.