Are "Comic Magazines" considered comics?

[Morsoth]

I mean, are magazines like Heavy Metal, image+ and even MAD are considered comics?

If yes, do they have any real value and be graded by a company like CGC?

I only started to collects image+ with the beginning of Wytches: Bad Egg by Scott Snyder and then I was asking myself if the people that bought the very first image+ with Here's Negan #1 have a gem in their hands?!... or if that first "issue" is worthless...

[TotalSnorefest]

They're generally more often referred to as 'anthologies', since most comics magazines tend to be compilations of several individually produced comics repackaged into a single product (magazine). Recent examples besidest the one you mentioned include Dark Horse Presents, Island Magazine, Drawn & Quarterly, Creepy, Eerie, 2000 AD and a bunch of Marvel/DC magazines like Tales of Suspense and the Witching Hour -- some of them will be just several comics with ntohing else, others include prose articles and other non-comics material.

In general I'd say they're all considered to be 'comics' regardless, or maybe 'comics commentary' when it comes to things like Wizard Magazine and Bleeding Cool's magazine.
Haven't the faintest if they're worth anything more than regular ol' comics. I highly doubt it unless you find a willing buyer... In regards to the Image+ example you mention, I doubt the first issues of that will ever become collector's items unless the Here's Negan pages never get reprinted.
Judging by the history of The Walking Dead, I see zero reason to expect this though - they'll presumably be included in some sort of collected volume or giant hardcover edition. Can't imagine why they'd leave those marooned in something so specific as Image+.

[Dark-Flux]

I wouldnt say Image+ is a comic necessarily in that its purpose is to mostly publish articles and interviews. Though its definitely featured comics and previews within it.

Heavy Metal is a comic, yeah. Or at least a collection of. And CGC do grade 'em.