First off, I LOVE the new ability to link your gmail account. This is extremely helpful, and removes 90% of my reliability with Gmail as it's all available with Proton Mail.

However, organization is needed for a couple different reasons.

1) Spam/Promotion
Gmail gives you HUNDREDS of emails into a quick "Promotions" area. All of this goes unfiltered upon linking with Proton, and floods your entire Proton Inbox.
For emails listed under "Promotion" tab in gmail (as well as other misc areas such as Spam), it should be filtered into some type of Subfolder and/or ignored.

I think the easiest way to do this is by asking simple questions upon linking the account.

"Would you like to include your Promotions folder? Y/N" - If you select no, the emails listed under the promotion tab won't be listed into your Proton Inbox, however all incoming mail from the Promotions Filter will still show up in the inbox.

That, or simply create a subfolder in Proton automatically titled "Gmail Promotions", and have the imported emails from such a tab go into there instead. This folder could go under the already existing folders feature, and can even be limited to Proton Subscribers.