I have valid gate score in 2018. so acc to IIT Gate score is valid for 3 years. In 2019 m.tech admission, i will get admission on gate 2018.

Since Gate score's are valid for 3 years, from the time of result commencement yes you will get admission in mtech 2019 based on the score of 2018,If any PSU comes out with notification for job vacancy in eg 2019 and asking for GATE score 2019 and 2018 then you can apply.You can apply for any other jobs for next there years with same GATE marks.