The GPL permits commercial use. What it doesn’t allow you to do is to supply a binary (commercially or otherwise) then refuse to the supply the source (except for parts that come from libraries that are normally found on the target system) or charge more for the source than the cost of physically providing it. Nor does it allow you to impose a condition that the binary not be further distributed, or only be further distributed if you are paid a fee to allow it.
The GPL is intended to encourage modification and even the re-use of fragments of the code in a completely different application.
The GPL considers operation of a service to be internal use, and imposes even less constraints. You are not even obliged to provide your service users with copy of the code or any modifications you have made.
There is a lot of material on the web about the GPL, although fsf.org is the definitive place for finding out the intent of the GPL.
Which licence applies depends on how you obtained the code. If you obtained it in dual licensed form, the licences that don’t conflict with your use remain in force. If you conflict with the GPL, all rights under the GPL are voided, but that doesn’t affect any other licence you obtained at the same time.