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 main plugin zip or as in Additional Gateway addon will not bring any difference or delay in loading time at all.
Both Digits and WPNotif plugin are highly optimized to just 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 Additional Gateways addon, but that won’t cause any difference.