My supplier sends me invoices via Gmail. How can I get them automatically forwarded?

That's one of our biggest use cases! You can refer to this link for a comprehensive guide by Google here.

Generally, the following screenshots should help you get set up sufficiently. 

What you need to get done:

1. Set up a filter for your emails
2. Set up automatic forwarding for these emails.

Step 1: Click on the Search Bar 

Step 2: Search for Documents via Keywords or "From" field, Click "Create filter"

Step 3: Add forwarding address under "Forward it"