Saying "I have to go" in chat
This is just fine, and it's a very native usage. :)

「得」 has the meaning of 'must' when read as 'dei3' in pinyin or 'ㄉㄟ ˇ' in zhuyin.

For expressing this, you could also say:


  
  我不得(note: 'de2', not 'dei3')不走了。
  我必須走了。
  


But '我得走了' is better in speaking and the above two are more used in writing.