रेलफैनों की तो बात ही कुछ और है

Exact differences shall be visible after the Tejas service actually rolls out. I think major difference between the two shall be
(1) deployment of on board train host/hostess (in Gatimaan) during the journey.
(2) Gatimaan was developed for speed with an aim to present it as alternative to Airlines and is supposed to be first Semi High Speed Train of IR. Tejas is just a more luxurious version of Shatabdi with a bit higher fare (about 10%).
(3)...
more...

Operational Speed- Gatimaan 160kmph (and track was specially upgraded for the purpose) Tejas 110/130kmph (no track upgradation done for Tejas)
(4) Avg. Speed- Gatimaan is only train on IR network with an avg speed of 100+kmph. Tejas trains fare very badly in this respect with avg. speeds in the range of just 60-85 kmph.
(4) Livery indeed- which matters the most to the onlookers, here Tejas wins with distinct bright orange livery as compared to little bit dull & boring Shatabdi like livery of Gatimaan !
(5) One more basic difference- Tejas is series of trains where as Gatimaan is likely to be a name of currently the fastest train of IR.