Dependency on the php_gmp module used to be one of major problems of PrivMX WebMail in versions 1.0.*.
This very important PHP module provides relatively fast mathematical functions related to encryption and decryption algorithms. However, many web hosting providers do not support this module(!) and we’ve been forced to think about different solutions. Since version 1.1.0, PrivMX WebMail has included also support for php_bcmath module which is a lot slower in mathematics, but also is a lot more popular among webhosting services. The both modules are being used via wrapper code which calls the fastest available functions.
The warnings during installation just indicate possible decreased performance of PrivMX because of missing gmp module.
Source code of the wrapper is availabe here: https://github.com/simplito/bigint-wrapper-php