Texting "GM" as a response to "good morning" is generally not considered bad or rude. In the world of texting and instant messaging, people often use abbreviations and shorthand to save time and make the conversation more efficient. "GM" is a common abbreviation for "good morning," and it's widely used in casual communication.
It's essential to remember that texting styles and preferences can vary from person to person. Some individuals prefer to use complete sentences, while others prefer to use abbreviations and emojis. So, if the person replied with "GM," it's most likely just their preferred way of responding and not a sign of any negativity towards you.
Unless you notice any other signs of discomfort or negative behavior, there's no need to worry or be paranoid about such a response. People's texting habits differ, and what matters more is the overall tone and content of the conversation rather than specific abbreviations or short responses.