Hoon, 29 bytes
|=(@ (reel (gulf [1 +<]) mul)
Hoon's native number is a bignum, so it works fine with 125 (or even 2000). It also correctly gives 1 for 0.
It uses +< in order to access the sample of the gate. This is axis navigation syntax: It means to access the tail of the subject, and then the head, which is where the sample is stored in the binary tree model Hoon uses.
Urbit drops you into a shell and Hoon REPL when you start it, :dojo. To test this, simply enter %. 125 on one line and then the snippet for 125!
Note there are two spaces between the dot and 1.
