"I have multiple Gmail accounts opened on my Android phone (Samsung Galaxy S6), and I no longer need them. So, is there any way for me to delete some of them?"
There are two methods for you to remove account. One is to reset your phone and the other is to delete your account. The former will clear all your files in the internal storage, and is not highly recommended if you just want to delete Gmail on Android. Here, we will mainly focus on the latter one.
The steps of the second way go like this: Settings > Accounts > Google > Choose your Google account > Remove account
Now I will walk you through the details about removing account from Android.
Step 1 Go to "Settings" > "Accounts". Select "Google" and select the account you want.
Step 2 Tap the menu icon. Select "Remove account".
Step 3 Click "Remove account" on the pop up and confirm to delete your Google account from Android. Note that removing the account will delete all of its messages, contacts, and other data from the device.