I've heard alot about Gmail been breached and hacked... It only proves that their security system is very bad.. they probably might have gotten a verification code through your mobile number...

most times you have additional security tips to help you Futher increase your security, and 2fa is just one of it... You can as well add a pass key, or Google prompt, that way when you wish to sign in, it simply ask you to click confirm on another device you last logged in your Gmail on..