@jabuci if you want specifically GMP, see fork of nim-gmp\:
https://github.com/subsetpark/nim-gmp
The old nimble package `nim-gmp` is abandoned, use `nimble install gmp`.

There's also a pure Nim bigint library from Status\:
https://github.com/status-im/nim-stint
And if you need this for cryptography, look into specialised libraries like
nimcrypto / constantine / openssl.