In order to send an email to a list of people you have in a spreadsheet, open that spreadsheet (e.g. Excel, Google Sheets) and highlight the email addresses. Do not highlight the heading if applicable.
Press Ctrl+C (PC/Windows) or Cmd+C (Mac)
Open a new email in Gmail. Click "BCC"
Press Ctrl+V (PC/Windows) or Cmd+V (Mac)
Continue composing your email as usual.