Good question; beware of any promotion that offers "free" anything. There is always a catch. Still it does appear that if you sign up you will be able to "purchase" up to $300 worth of services within a certain period. In other words you can use the services during the period of the free trial but, as as the "Supplemental Terms and Conditions For Google Cloud Platform Free Trial" state at 4.1, you cannot use the services you "bought" after the free trial period unless you become a paying customer and upgrade to a full account. It is just another way of giving you a free trial but dressing it up as something of value by expressing it in $. It is a little misleading in my view.