You should receive a courier message after you hit Lv20, inviting you to see the museum. But you can just head there anyway after you reach Lv20, you do not need to wait.
The museum is in Dawnstar, Skyrim.
He lives in Dawnstar.
On the coast, northeast of Dawnstar.
You have to go through the Dark Brotherhood questline. Innocence, my brother.
In the "Dawnguard" DLC for Skyrim, Gunar can be found in the town of Dawnstar. He is typically located in the Windpeak Inn, where he can be seen sitting at a table or interacting with other NPCs. If you're looking for him for quests or dialogue, checking the inn is your best bet.
You need to complete the 'Walking Nightmares' and then the 'Kill Giants' quest for Dawnstar. Then you will need to help three Dawnstar residents to become Thane. It is the only house in the Hearthfire DLC where you are required to become a Thane before you can purchase it.
The Tower of Mzark is located in the Pale, south of Dawnstar. Source: Elder Scrolls Wiki page "Tower of Mzark"
The nine holds of Skyrim are; Eastmarch (Windhelm) Falkreath Hold (Falkreath) Haafingar (Solitude) Hjaalmarch (Morthal) The Pale (Dawnstar) The Reach (Markarth) The Rift (Riften) Whiterun Hold (Whiterun) Winterhold (Winterhold)
He opens it on Saturdays 9:00 am through 9:00 pm
To refill the vendor chest in Dawnstar in Skyrim, you need to wait for 48 in-game hours after the vendor's stock has been depleted. You can do this by resting at an inn or using the wait function. Once the time has passed, the vendor's inventory will refresh, and you'll find new items available for purchase. Keep in mind that some items may be randomized each time the vendor restocks.
to my knownledge there is no workbench in dawnstar
Dawnstar was created in 1977.