when the implied odds are high enough. If you are only going to hit the card 8% of the time, which is like 4 outs or so then chances are you should almost never call for the gutshot, But if the implied odds in the hand are big enough then you can justify it. If they have plenty of chips behind and you put your opponent on a big hand and you have plenty of chips behind then there is plenty of value to gain from hitting your hand, granted that the bet isnt too big of your chips stack or of the pot size, Then you will have sufficient pot odds to call on hitting your gut shot because the payoff on the river should yield big returns, usually their entire stack or a great deal of money. This is the only time i will call gutshots, if my opponent doesnt have many chips behind or not very much money there is no point to me calling to hit the gutshot unless the pot odds are good, which they usually aren't. If you are 8% to hit the gutshot then the bet would have to be very low to the size of the pot for you to call and hit without any implied odds. Only call if you have a lot of value to gain in the hand after your card has hit, that is the only time you should call a gutshot. Good luck.