I've pulled the Omnipay GlobalIris payment gateway from the repo shown below and added the code to the third_party/store/vendor/omnipay directory in Expression Engine - where there are already a number of other Omnipay gateways installed.
https://github.com/maclof/omnipay-global-iris
I can't figure out how to go about completing the installation and adding the GlobalIris gateway to the others - do I need to edit a composer.json file somewhere and then run composer?