Same issue with ubuntu instance.
I can see the gmp module through
/opt/plesk/php/7.3/bin/php -m
Ran following commands
sudo apt install autoconf automake gcc libhashkit-dev pkg-config plesk-php73-dev zlib1g-dev - worked fine
then ran /opt/plesk/php/7.3/bin/pecl install gmp
this gave following...