There's this web app licensed under GPLv3 that I'd like to deploy. However, the footer of each page has a copyright line, e.g., "© 2015 FooBar Inc.". Is it okay to remove that line and/or replace it with one relevant to my application, e.g., replace it with my entity?
Actually, you probably can remove because, you are not distributing the software so the GPL doesn't affect what you do. Deploying a web app implies that your users will interact remotely with the software, as opposed to receiving a copy of the software itself, so as far as the GPL is concerned, you are merely using the software privately.
Had the license been AGPL instead, the answer would be different.
GNU's FAQ contains a question on this issue:
A company is running a modified version of a GPL'ed program on a web site. Does the GPL say they must release their modified sources? (#UnreleasedMods)
The GPL permits anyone to make a modified version and use it without ever distributing it to others. What this company is doing is a special case of that. Therefore, the company does not have to release the modified sources.
It is essential for people to have the freedom to make modifications and use them privately, without ever publishing those modifications. However, putting the program on a server machine for the public to talk to is hardly “private” use, so it would be legitimate to require release of the source code in that special case. Developers who wish to address this might want to use the GNU Affero GPL for programs designed for network server use.
The line "© 2015 FooBar Inc." is a so-called copyright notice. The GPL has the following to say about notices in case you modified it:
5a: The work must carry prominent notices stating that you modified it, and giving a relevant date.
This doesn't say explicitly that you must change this line or add to it, but changing this line would definitely make this clear.
You are allowed to remove it per the license (but not necessarily in your jurisdiction; copyright notices may have separate laws governing them) if there are no other restrictions, but the GPL does allow additional restrictions on copyright notices:
7b: [you may require] ... preservation of specified reasonable legal notices or author attributions in that material or in the Appropriate Legal Notices displayed by works containing it
This seems to be a "reasonable legal notice", so it is possible that the license you received requires you to keep it there.
In either case, you should note near the original copyright notice that you modified it, possibly with a copyright notice of your own.
If it hasn't got such a requirement, it may be more useful to move it to a central place which lists the copyright holders - a notice file or about -> copyright menu item in interactive applications for example.