Google gives me 15 Gb of free space, to be used in Google Drive, Gmail and Google Photos. Problem is, it got 99% full. I realised that all these email newsletters added up to several Gb of storage used for nothing. This URL was very useful to understand where the problem was:
https://one.google.com/u/0/storage
To get a few Gb back, you could filter on Gmail per email size, but that did not help much. The bigger problem was these companies sending me hundreds of messages, a daily newsletter for example. So what I did was to go the latest emails I receved on the categories Promotions, Updates, Forums, unsubscribed from the senders, and then selected all the messages from these senders, and deleted all of this.
Select all conversations, then click on the trash icon…
Then I of course had to empty the trash…
In total, thousands of messages, and about 2 Gb deleted, it felt quite satisfying.