Earlier Indian Railways had planned to convert all Shatabdis into T18 and All Rajdhanis into T20. But because of political interference It was sent to Varanasi and Katra instead of Pre planned Bhopal Shatabdi, Chandigarh Shatabdi and Lucknow Shatabdi route.
I think things have not changed much yet and now T18 rakes should be given to All Shatabdis, started with Bhopal, Lucknow, Chandigarh, Ahmedabad, Banglore Shatabdis.
Gatimaan rake can be changed with Tejas rake. Though it is the fastest running train in India...
more... and still doesn't have a different livery of coaches.
Changing Gatimaan into T18 won't be a good idea because as per new guidelines if railway a T18 between Chandigarh and Jhansi via Delhi, and Delhi Bhopal is already planned.
Even existing Gatimaan Express can be Easily extended to Chandigarh because of Upgrade of MPS of track is already sanctioned.