Isn't Google's $300 credit only valid for 2 months?
In addition, the $300 is only for expenses in excess of the "Always Free" usage limits, which covers quite a bit of stuff, including 5 gigs of cloud storage, 1 micro compute engine instance, and a terabyte of BigQuery queries per month.
(disclaimer: I work at Google)
It absolutely used to be 2 months, otherwise, my account would still be able to use those 300$ credits. But I just checked, it can't. I still have the expiration email if you want.
Building on google stuff is as risky as building on apple's.