I currently want to improve an application which is licensed under GPLv3. I wanted to improve it's design and Interface and use icons from google which is licensed under Creative Commons Attribution 4.0 International Public License. On Gnu website it seems like it's compatible as I understand. But because of this line it should not be used on software. So I wanted to clarify If I can use them together
The CC-BY 4.0 license is a very permissive license. The only requirement on you is that you give proper attribution where you got the icons from.
The Creative Commons licenses are designed to be used on artwork, literary works and the like. These licenses lack some considerations that are specific to software (such as the distinction between source code and (derived/compiled) binary code) that makes these licenses less suitable for use on software.
The caution "it should not be used on software" means that you should not write software and release that under on of the CC licenses, but it does not affect how you can combine artwork (such as icons) that is already under a CC license with a separate software product under a different license.
In particular, if you modify an existing application that is under the GPL license, then the software will always remain under the GPL license. Using icons that are licensed with CC-BY does not change that. The CC-BY license is also permissive enough that you can release the modified program with the new icons entirely under the GPL license.