Vim is able to query tags from multiple projects and usually all you need to do is set the
'tags' option to point to the
tags files in each directory.
'nocompatible' mode (which is the default),
'tagrelative' defaults to "on", which means when Vim finds the tag in a
tags file in another directory (a tag belonging to the other project), it will look up the file name relative to the directory where that
tags file is, rather than the current directory.
So you could also look up tags from the Linux kernel source tree using:
/path/to/src/linux is the directory where your Linux kernel source tree lives, and that you have successfully created a
tags file there. You can do so by changing to that top directory and running
ctags -R . (but the exact details might depend on which version of the "ctags" program you have in your system.)
I'm not really sure how plug-ins such as "gutentags" or "gtag-cscope" will interact with these settings. My understanding is that they're mostly concerned with keeping the
tags file up to date, by invoking
ctags in background after you modify local files, so I think it's quite reasonable that they'll just leave your addition to the
'tags' option alone, so it's quite possible everything will just keep working even in the presence of such plug-ins. So I'd suggest you just go ahead and try it, see if it works, it's likely that it will.