how much is garry's mod on steam trades?

Stop being lazy and go look on SteamTrades yourself. The best way to see what something's worth is to look what most people are selling it for.

Go there and check it? lol?

A key = somewhere between about $2.00 and $2.40 or so, depending on how you choose to look at it.
Garry's Mod will generally cost you 1 key when it's on sale, 2 keys when it's off-sale. Probably won't go any higher than that unless it hasn't been on sale in a really long time.