How do I get access to all GPU shapes in DevTest Labs?

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Happy to answer your questions

Repro:

If you go to your DevTest Labs resource, click "Add" to add a VM, choose a base, and then click on "Change Size" under "Disk and Size" for your VM size settings, you'll be presented with a list of VM options. By scrolling down to the bottom you'll see the N-series VMs which are grayed out with the following message:
"Size not available. This size is currently unavailable in this location for this subscription: NotAvailableForSubscription."

Solution:

It turns out that for this size family, the default vCPU (core) quota for your subscription is initially set to 0 for each region. You'll need to request a vCPU quota increase for this family (and make sure that it is in an available region).
On the "Request a vCPU quota increase" page there is a vague instruction to make the request using the portal. What this really means is:

• Go to "Subscriptions" ("All Services" > "Subscriptions", it will have a key for an icon)
• Select the subscription you want the quota increase to be applied to
• Click on "New support request" under "Support + troubleshooting"
• Complete the wizard:
  Issue Type: Service and subscription limits (quotas)
  Subscription: Your subscription
  Quota Type: Compute-VM (cores-vCPUs) subscription limit increases

Hope this information helps.

Sourced from MSDN