Gmail + OpenAI AP Invoice Forecast n8n to Google Sheets

Turn invoice emails into automated cash-flow forecasts—syncing to Google Sheets with OpenAI

This n8n workflow monitors your Gmail for unread invoice emails with PDF attachments, extracts invoice details using OpenAI, logs everything in Google Sheets, calculates upcoming cash needs, and sends you Gmail alerts—including a weekly accounts-payable digest.

What this workflow does

• Detects incoming invoices: Triggers when an unread Gmail message arrives and includes a PDF attachment, then downloads the invoice file.
• Extracts invoice data with OpenAI: Reads the PDF text and uses OpenAI (GPT-5-mini) to extract structured invoice fields, categorize the expense, and generate a one-sentence summary.
• Normalizes and prioritizes: Structures the extracted data, assigns High or Normal priority based on the invoice total, and records sender and receipt timestamp.
• Logs to Google Sheets: Appends each invoice record to Google Sheets, then calculates month-to-date spend and amounts due in the next 30 days.
• Sends Gmail notifications: Emails cash-flow metrics after each processed invoice, and sends a separate approval-style alert when priority is High.
• Delivers weekly AP overview: Every Monday at 9am, reads the full invoice log from Google Sheets, builds an HTML weekly digest (totals, category breakdown, and invoices due in 7 days), and emails it via Gmail.

Use cases

• Accounts payable teams who want invoice parsing and tracking without manual spreadsheet updates.
• SaaS operators forecasting cash by automatically estimating due amounts from emailed invoices.
• Automation engineers setting up an invoice-to-forecast pipeline in n8n using Gmail + OpenAI + Google Sheets.

Technical details

• Integrations / nodes: Gmail Trigger, Gmail, OpenAI (Chat Model, default gpt-5-mini), Google Sheets, plus workflow nodes such as if, set, code, and sticky note.
• Auth required: Gmail OAuth2 for both the trigger and outgoing email actions; OpenAI API credential for invoice extraction; Google Sheets OAuth2 for logging and forecasting.