Am I allowed to run some tests I created on Gmail or are there any policies against this? This is purely for practice.
Answering your Question straight Yes you can practice automation testing on Gmail.
Having said that there are certain Caveat mentioned in the Google Terms of Service. The most important of them being the first one as:
Don’t misuse our Services. For example, don’t interfere with our Services or try to access them using a method other than the interface and the instructions that we provide. You may use our Services only as permitted by law, including applicable export and re-export control laws and regulations. We may suspend or stop providing our Services to you if you do not comply with our terms or policies or if we are investigating suspected misconduct.
Its worth to mention that Google extensively uses cookies and finger prints (to name a few) to identify an user. Somehow if your IP gets black-listed it will be tough task to regain back the access.