Just want to make sure I am not missing any games of this type. Basically, I want to know whether there are any period Dungeon Master (DM) like games on GOG that I am missisng.

Basically, to count as a DM-like game, the game needs to meet these requirements:
* First person view, with no separate screen for combat
* Real-time combat, though attacks may have cooldowns before you can attack again
* Grid-based movement (so games like Ultima Underworld are out)
* Game is primarily mouse controlled (though using keyboard for some functions is acceptable)

But I am also looking at games that
* Are available on GOG (so no Dungeon Master as of this post)
* Were released during the time such games were initially popular; in particular, I could require that such games be released before the year 2000. (So, Legend of Grimrock is out.) (This is the "period" part of the topic's subject.)

So, what I currently have:
* Eye of the Beholder Trilogy
* Dungeon Hack
* Legends of Lore 1 (2 and 3 aren't grid based, so they don't count)
* Stonekeep
* Anvil of Dawn

Are there any others on GOG?

Ravenloft: Strahd's Possesion
Ravenloft: Stoneprophet
Menzoberranzan

All of them have free movement, but they are still built on a grid and in the options you can actually set to enforce grid based movement if you want. Otherwise they play pretty much like Eye of the Beholder but more open worldish.

We need Dungeon Master 1 and 2 on GOG!

Ishar Trilogy is another one you can add to your list.

Bloodwych (and its sequel, Hexx: Heresy of the Wizard) would be another one which hopefully will be available on GoG one of these days.

Obligatory link to the wishlist entries:
https://www.gog.com/wishlist/games/bloodwych
https://www.gog.com/wishlist/games#search=hexx%20heresy

The Realms of Arkania series (based on the German Dark Eye RPG system) may be worth a look if you're willing to relax the "separate combat screen" restriction:

https://www.gog.com/game/realms_of_arkania_1_2
https://www.gog.com/game/realms_of_arkania_3

For an in-depth review of the first game, see:

https://crpgaddict.blogspot.com/2019/11/game-348-realms-of-arkania-blade-of.html

You mean Lands of Lore: The Throne of Chaos. But yes, apart from the Ishar games mentioned by truhlik, I think you've found all that fit your specific requirements.

We need Bloodwych and Hexx: Heresy of the Wizard here!

The Legacy from Magnetic Scrolls/Microprose meets the requirements. Dungeon Master style RPG Crawler in a labyrinth haunted manor, real time combat, magic, puzzles, high difficulty, Character creation etc...

EDIT: Oh, and I forgot. Right now in the Piko weekend sale

There's Grimoire also - released far later but pretty much a 2000-style UI. Has a somewhat controversial developer though:

https://www.gog.com/game/grimoire_heralds_of_the_winged_exemplar

https://www.rockpapershotgun.com/2017/08/09/grimoire-the-rpg-20-years-in-the-making-is-a-crazily-tough-nut-to-crack/

Yep, that's a good one too.

Captive (and its sequel, Liberation: Captive 2) as well as Hired Guns would fit the bill too. Both aren't available on GoG yet, though.

Wishlist entries:
https://www.gog.com/wishlist/games/captive
https://www.gog.com/wishlist/games/captive_ii_liberation
https://www.gog.com/wishlist/games/liberation_captive_2

https://www.gog.com/wishlist/games/hired_guns

Not fitting the movement and/or the combat mode criteria AND available on GoG:
https://www.gog.com/game/might_and_magic_6_limited_edition
https://www.gog.com/game/the_bards_tale_trilogy
https://www.gog.com/game/wizardry_6_7
https://www.gog.com/game/wizardry_8
https://www.gog.com/game/betrayal_at_krondor
https://www.gog.com/game/superhero_league_of_hoboken
https://www.gog.com/game/an_elder_scrolls_legend_battlespire (incl. Daggerfall and Arena)
https://www.gog.com/game/thunderscape
https://www.gog.com/game/albion
https://www.gog.com/game/strife_veteran_edition
https://www.gog.com/game/wizards_warriors

Not fitting the movement and/or the combat mode criteria AND NOT available on GoG (yet):
Alternate Reality series
Alien Fires 2199 A.D.
Wizardry 1-5
Circuit's Edge
Moraff series
Fate: Gates of Dawn
Amberstar
Ambermoon
ShadowCaster
Witchaven
CyberMage: Darklight Awakening
Mordor: Depths of Dejenol
Descent to Undermountain
Black Crypt

Indeed we do.