If he's all in on the flop for 250, you are getting two shots at it, so it's as if you have 8 cards to win, so your odds of hitting are twice as good (making it a call).

Obviously this is theoretical - if you felt like you could get him all in for a pot size amount if you hit the gutshot and he covered you, then you might call with the original odds you described. But that's another discussion and is situation-specific.

In fixed limit games, there are occasions in which chasing a gutshot is worthwhile. (e.g. multiway pot with no flush draw, and you're on the button with the guaranteed nuts if you hit the straight) but it's almost always wrong to chase a naked gutshot (no overcards) in no limit games.
With a gutshot, you have a less than 1 in 10 chance of making the straight on the turn. With two cards to come, your odds improve to about 5:1 (a 1 in 6 chance), but you'll probably have to call another bet on the turn, by which time villain could have improved his hand further, or picked up a flush draw, killing one of your outs.
Even if villain only bets half pot on the flop, you simply don't have the right odds to call with a naked gutshot.
However...
The thing with no limit is that villains will often c-bet (semi-bluff) the flop with air (e.g. ace high), so if you were chasing a gutshot with JT on a 9K2 flop and hit your your Jack on the turn, you will have missed your straight but made a better hand than villain's ace high. Your skill in hand-reading and understanding villain's range is what makes odds calculations somewhat irrelevant in no limit. Until you get better at understanding ranges and reading your opponents, then you are probably better off dumping gutshot draws on the flop if villain shows any sort of strength. Chasing gutshots is strictly for the fish.

So then what rule do you use if you are not in an all in situation on the flop? Just use the rule of 2?

confused here..

Yes, if you don't have other information. When you know you will be facing a turn bet only half the time and getting a free rivercard the other half, then you could use the rule of 3.

Or am I confusing you even more?

I see...and I'm guessing you'd know the turn bet frequency with a stat on hud?

Bump for future linkage!

shick is god. nice, thanks for making it plain and simple.

I have been reading, and i would really like to know, from where are these numbers coming? I mean, why is it the rule of 2/4 and no 3/6 or something like, why the 2 and the 4?

Sorry for asking, maybe it seems too obvius, i understand it, but i just wanna know from where it comes mathematically. Thanks.

It derives from the fact that on the flop there are 47 unknown cards and on the turn there are 46 unknown cards, which you would use for an exact calculation. Both numbers are close to 50, which translates to 2% for each out.

Example: On the turn, if you have X outs, the exact probability of a hit is X/46. If X = 9, the exact probability is 9/46 = 19.57%. The 2x rule is 2*9=18%, the same as 9/50.

A similar but more complicated analysis applies for the 4x rule on the flop with 2 cards to come.

Many thanks for your answer