If I look at the second test data sample provided in the Elixir version:
lst = [ [690, 1300], [87, 1310], [30, 40] ]
testing(lst, [ [18078, 34060], [2262, 34060], [25545, 34060] ])

The GCD of 34060 is incorrect. 34060 / 1300 = 26.2, 34060 / 1310 = 26, and 34060 / 40 = 851.5. I did all the prime factorization by hand, checked my algorithm for LCM, as well as verifying with other sources, and the correct GCD should be 340600.

Since it didn't specify UTF-8 chars, you're absolutely right. In that case, I would probably use String.graphemes() instead

Try to avoid mutable variables in F# when possible. For this example, the explicit mutation is not necessary.

It's not necessary to declare min and max as variables. The final statement in an F# function will evaluate as the return value for that function

Will do! I should have one together fairly shortly.

Float.to_string/2 is now deprecated and generates a compiler warning