This is a fake message. There are no government orders on GST being imposed on Whatsapp messages.
In January 2018, The Wall Street Journal published a report on the number of good morning messages sent in India. The report revealed in the last five years, there had been a 10-fold increase in the number of Google searches for images for wishing 'Good Morning'.
In March 2018, an image of the clipping of a newspaper article was circulated in social media. It stated that the Government of India would impose 18% Goods and Service Tax (GST) on sending and forwarding good morning messages on WhatsApp from the following month.
It was found that the article was from Navbharat Times' March 2, 2018, Delhi edition. The story carried the advisory 'Bura na mano Holi hai!' (Do not feel bad since it is Holi.), implying that the article was published in jest by the media organisation on the occasion of Holi.