It appears to be installed on my shared server M**** (I haven't tried using it, but it appears in phpinfo()). Is this something recent? My server is at PHP 7.2.19 -- perhaps yours hasn't been upgraded yet, or you forced it to a lower level? Perhaps the next step is to open a support ticket and see if it is supposed to be on your server soon?
I'm not sure why GMP (an extended precision library, also known as GNU MP) would be on a web server, or what use it would be (particularly for phpBB). It sounds like something that could suck up a lot of CPU cycles, and have a negative impact on other customers, so I'd be surprised if they installed it. I assume you aren't talking about some sort of library related to GIMP, the image processing application. That sounds more like something phpBB might use.