List of Gateways can be seen here https://digits.unitedover.com/supported-gateways/
Will this increase loading time?
Additional Gateways addon is very lightweight and is totally optimized to use just what is needed. So having the gateway in the main plugin zip or as in the Additional Gateway addon will not bring any difference or delay in loading time at all.
Both Digits and WPNotif plugins are highly optimized to use the files that are needed for functionality, so only the gateway(s) you use will be loaded everything else will not affect anything.
Can I remove the gateways I don’t need?
Yes, you can definitely remove the gateways you don’t want from the Additional Gateways addon, but that won’t cause any difference.