estimation:

• 0,500s to reach N==5000
• 100 random tests
• overall, around 50s, let say 25 because all inputs won't be as big as 5000. Meaning, way too much time needed to not time out.

Now, since you won't be able to make the generation faster, think about what you need to actually pass the tests. :)

Specially this part:

gap(6,100,110) --> nil or {0, 0} : between 100 and 110 we have 101, 103, 107, 109 but 101-107 is not a 6-gap because there is 103 in between and 103-109 is not a 6-gap because there is 107 in between.

More than 5000 guys passed the kata... Read the description again.