Google provides access to Gemini via Google Cloud / AI Studio. Use the links and steps below to obtain a key and configure the app.
Google often offers a free trial for new accounts
Use that account to create and test your Gemini API key.
The screenshots below follow the same order as the steps above. Files are named 1.png, 2.png, and so on.
Step 1: Sign in to Google Cloud or AI Studio. Google often offers a free trial for new accounts.

Open Google AI Studio and sign in with your Google account.
Step 2: Enable the Generative AI (Vertex AI / AI Studio) APIs in the Cloud Console.

Enable the Gemini-related APIs in Google Cloud.
Step 3: Go to APIs & Services → Credentials and create an API key.

Create a new API key in the Credentials area.
Step 4: Copy the generated key before leaving the page.

Copy the generated Google API key.
Step 5: Open Settings and select Google Gemini as the provider.

Switch the app provider to Google Gemini.
Step 6: Paste the key into the API Key field.

Paste the Gemini key into the app field.
Step 7: Save the settings so the app can use the key.

Save the settings to finish setup.