Linux is the kernal, the bit of the OS responsible for interacting with hardware and organising processes that run
GNU packages are a collection of small software programs that are present in virtually every Linux distro (things like bash, grep, and libc) that are needed to make the OS do useful things.
together they make up “GNU/Linux”, which is often shortened to simply Linux. Which is fine, people will understand what you’re talking about from context, and only pedantic asshats will correct you in general conversation. This is the internet though and you’re bound to run into a pedantic asshat from time to time.
The long and short of it is this: