Earlier quoted context omitted.
Except: "Sorry, you aren't eligible for a free trial at this time. The free trial is for new customers only." Apparently, the fact that I've been curious enough to experiment with other Google developer products in the past means I'm not part of the target audience.
Sorry about that! Free trials have a timeout :( Can you submit a support request and we'll see what we can do? Also, spinning up a cluster should be incredibly cheap if you just want to mess around for a little bit - we do billing by the minute :) Full disclosure: I work on Google on Kubernetes
"Unfortunately, the system is developed by design to only apply the free trial credit to new email address creating a new billing account and we can't apply it for already existing emails." Bummer.