Be careful with Gmail: Google openly acknowledges that they scan emails sent through their service in order to use your data for marketting purposes. Given the increased societal awareness of suicidality, there's a chance their algorithms may flag your email and bring it to someone's attention. Facebook is very proud to have rolled out an algorithm that flags possible suicidal posts --I posted the article on it here several months ago; I'm sure a websearch will turn up more info, too-- and I wouldn't be surprised if Google is doing the same in the name of "social responsibility."
The big question then is when does Google scan your emails? If it is only when they are sent, there should be no trouble: by the time your delayed email is sent to the police, you'll already be dead. However, if they scan emails that are still in the queue to be sent on delay, you may have problems.
To that end, I use LetterMeLater for delayed emails: it's independant of Gmail, so I like to think they don't scan. I've used them for a couple of attempts --and cancelled the emails in the queue-- without repercussions, so can confidently recommend them for the purpose. I know emails sent through them reach the local police, too: I sent test emails via them on a couple innocuous subjects --asking about reporting illegal fireworks use, I think-- and my emails apparently went through just fine. Some email-delay services get flagged as spam, and your email never arrives.
DO NOT use Timecave: not only did their services get most of my emails flagged as spam (though not the test emails to the police), they read my email to the police several hours after I supposedly cancelled and deleted it, and then forwarded it to the police! I had some fast talking to do when an officer showed up at my door for a "welfare check," after "information was found on my computer that implied I was suicidal." Everything ended well, but it was quite tense for a while. That episode was what inspired me to change from lurker to member here, so I could post my account of it as a warning.