Is there no warning if landing gear is not retracted during flight?
It's not there because it's like driving with the handbrake on.


  [The flight] was climbing out of Kolkata, however achieved an unusually poor climb rate, was not able to accelerate to normal cruise speeds and leveled off at FL200 after a continuous climb for about 30 minutes (avherald.com).


And because the flight crew should have run the 'after takeoff' checklist, which includes checking that the landing gear is up and locked.

(Source) A typical Airbus A320 checklist.



Related: How much extra drag does landing gear incur?