My car shows a gear sign on dashboard, it does not show any trouble codes on diagnosis

It may be just a temporary issue. Try this,

Put shifter in park, put the park brake and turn the engine off. Wait a few seconds and restart the engine. If the gear sign still shows, then it's a more serious problem than it looks.

 If such is the case then your transmission is probably slipping.

Drive the vehicle while trying to maintain a constant throttle position as it accelerates upwards thru all the gears. If the transmission is shifting properly, it should be in 4th gear by the time you reach 60 to 80 kph on level ground. Repeat the procedure 3 to 5 times Rescan the PCM for trouble codes, if nothing appears again then the problem could have been a onetime occurrence. If a trouble code appears you take it from there and fix as necessary.

Even if it resultantly appears to be a onetime thing,… that is not an incentive enough for you to trust that your transmission is okay. 

It owes partly to bearing, clutch or hard part failure, and this will come with a grinding, whirring noise but again, not every time.