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%).
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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.

* This livery was earlier considered for the Tejas group of trains, but later on the vinyl coated orange one was finalised.
* It may be noted that earlier considered livery for Humsafar (predominantly white) was also not so attractive, but later on the current beautiful sea-blue livery was finalised.
* Looks like RM/IR wants the new trains to be a treat to passengers as well as the onlookers!!
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Good thinking indeed. Attractive packaging without significant additional cost is always a good marketing strategy!!
* Suvidha (convenience), Swachhta (cleanliness) & Sundarta (beauty) are the three Ss emphasised by IR/RM. After Swachh(clean) trains, Swachh Rly stations, Swachh trains (on board cleaning), Suvidha (on-line on-board help through twitter) the Sundarta- Beautification of railway stations (paintings, Murals etc), beautiful exterior as well as interior of trains is a logical step.