First check that the URL for your Bitbucket Server / Bitbucket Data Center instance is publicly available with a public DNS record for Gearset to connect. You can verify this by entering the instance URL at https://dnschecker.org/

Initial Bitbucket Server / Bitbucket Data Center setup (team owner)

Then you'll need your Bitbucket Server administrator to create an application link within Bitbucket Server to Gearset. Then you'll need to get your Gearset team owner to link your Gearset team to that Bitbucket Server instance. This provides a mechanism for Gearset to authenticate against Bitbucket Server and only has to be completed once for your team. Once this initial connection has been made, any other users on your team can set up their own Bitbucket Server connection themselves in a couple of clicks.

Step 1: Copy Gearset's public key (Gearset team owner only)

In Gearset, under My connections, click the Source control and services page.

Click + Connect to Bitbucket Server / Bitbucket Data Center.
In the resulting dialog, get the generated Public key and Consumer key to hand as you'll need them for Step 2.

Step 2: Creating an application link in Bitbucket Server / Bitbucket Data Center (Bitbucket Server admin only)

In a new tab, sign in to your Bitbucket Server / Bitbucket Data Center instance as an administrator and navigate to the Administration page then click Application Links from the menu. (Note: this must be completed in a new tab to preserve the generated Consumer key.)

Click Create link and choose Atlassian product. Enter https://app.gearset.com in the box and click Continue.

When prompted with a dialog to confirm the application URL, click Continue.
The yellow warning can be ignored.

In the Link applications dialog, enter an Application Name, and leave the Application Type as Generic Application. It's important to leave the Service Provider Name blank and to check Create incoming link before clicking Continue.

In the Link applications dialog, enter a Consumer Name and Gearset’s Consumer Key and Public Key from Step 1. Once the configuration is complete, click Continue.

Once completed, Gearset should appear as an application link in your Bitbucket Server / Bitbucket Data Center instance.

Step 3: Authorize Gearset as your Bitbucket Server / Bitbucket Data Center user

Switch back to the Source control and services page in Gearset. In the same Add Bitbucket Server dialog as before, enter the URL of your Bitbucket Server / Bitbucket Data Center instance and click Connect.

You’ll be redirected to your Bitbucket Server / Bitbucket Data Center instance and prompted to log in, if you aren’t already. When prompted, grant Gearset permission to make changes to your Bitbucket Server / Bitbucket Data Center data by clicking Allow.

Once complete, you'll be redirected back to Gearset.

Connecting to Bitbucket Server / Bitbucket Data Center (team members)

Once the Bitbucket Server / Bitbucket Data Center application link has been created and Gearset has been authorized by the team owner, the rest of the team members will be able to connect to Bitbucket Server:

Navigate to the Source control and services page and click + Connect to Bitbucket Server.
Click Allow within the Welcome to Bitbucket Server dialogue box.

You'll be redirected back to Gearset and can start working with repositories on your Bitbucket Server instance.