By default, Gmail is in rich text mode.
If you want to use plain text mode, then you need to enable it.
Remember that enabling plain text mode will disable rich text mode.
Here are the steps to enable:
Sign in to your Gmail account.
Click on the Compose button at the top left.
In the New Message pop-up, hit the three vertical dots which you can find beside Trash can icon at the bottom.
In the menu, hit the Plain text mode option, which you can find between Label and Print.
Remember that a checkmark beside Plain text mode option means plain text mode is enabled.
Keep in mind that you can enable or disable plain text mode while replying and forwarding too.
Disabling plain text mode will enable rich text mode, and the above steps can do it too.