my gate score in 2017 is 483.I am going to appear for gate 2018.If i get a score less than previous score will my previous score be invalid?

It depends what you are applying for like M.Tech admissions to IITs and CCMT considers best score of both whereas if you want to go for PSU then it does not matter as it depends upon the particular score as per year asked by PSU like few of them recruit on the basis of current score and few of them accepts previous scores as well so it varies PSU to PSU. Hope this helps!!!