To begin troubleshooting, try logging into your Gmail account on gmail.com. If that doesn't solve the Gmail keeps crashing problem on your iPhone, you can proceed to solve the software-related issue. It's best to quit the application and restart it because apps that weren't closed or left suspended while an update was being installed usually crash or act up. As a second solution, restart your phone as it will also clear any errant apps and dump any corrupted data, including those that caused Gmail to fail to function. The third solution is to update your iPhone's Gmail app because applications that aren't updated will usually be the first to experience issues with updates.