You cannot use GPL-licensed code on a proprietary web frontend.
When you include GPL-licensed code into any software, that software becomes a derived work of the GPL-licensed code. Only the GPL gives you the right to publish the resulting software.
You may charge for the software or for services around the software. But once anyone receives the software, they obtain a license under the GPL to use and share this software. You cannot add additional restrictions. So any customer may start offering your software for free. It is therefore common not to charge for the software itself, but for services such as administration, customization, or hosting.
Note that your server backend is generally a separate software. If you use GPL libraries on your frontend, this does not put any requirements on the backend. If you do not want to publish certain code, run it on your backend and have the frontend make Ajax requests to the backend.