To upload contacts to Gmail, you import them into Google Contacts , which then syncs with your Gmail account. The key is to have your contacts in a CSV or vCard/VCF file first.

Basic method (desktop)

  1. Prepare your file
    • Put your contacts into a spreadsheet (Excel or Google Sheets).
    • Include at least: Name, Email Address, and optionally Phone Number, Company, etc.
 * Export or save the sheet as a CSV file.
  1. Open Google Contacts
    • Sign in to your Google account in a browser.
    • Go to Google Contacts (often via the 9-dot Google Apps icon in Gmail, then Contacts).
  1. Import the file
    • In Google Contacts, look for Import in the left sidebar (sometimes under Fix & manage or Contacts).
 * Click **Import** → **Select file**.
 * Choose your CSV (or vCard/VCF) file.
 * Click **Import** and wait for the contacts to be added.

After this, the uploaded contacts will appear when you compose emails in Gmail (auto-complete by name or email).

From another Gmail account

If your contacts are in a different Gmail account:

  1. In the old account’s Google Contacts:
    • Click Export.
    • Choose the contacts to export and select Google CSV as the format.
 * Download the file.
  1. In the new account’s Google Contacts:
    • Sign in with the new account.
    • Use Import and select the downloaded CSV file.
    • Complete the import to upload all those contacts.

On Android

  • Open the Contacts or Google Contacts app.
  • Go to the Fix & manage tab (or menu) → Import from file.
  • Choose your Google account.
  • Select the CSV or VCF file stored on your phone.
  • Confirm to import; the contacts sync to Gmail for that account.

On iPhone (VCF method)

  • Get a VCF file containing your contacts (for example, exported from another service).
  • Open the VCF file on your iPhone.
  • Review the contacts and tap Done / Import.
  • Make sure your iPhone is set to sync contacts with your Google account so they appear in Gmail.

Mini tip:
If your CSV columns are confusing, create one sample contact directly in Google Contacts, then export your contacts as CSV and use that as a template for formatting your bulk list.

TL;DR:
Upload contacts to Gmail by importing a CSV or VCF file into Google Contacts via the Import option; once imported, those contacts automatically show up in Gmail’s address suggestions.

Information gathered from public forums or data available on the internet and portrayed here.