Is there any reason to use only UTC standard time in aircraft and not GMT?
UTC is in principle the same as GMT.

But for accurate applications there is a difference:


  Saying "GMT" often implies either UTC or UT1 when used within informal or casual contexts. In technical contexts, usage of "GMT" is avoided; the unambiguous terminology "UTC" or "UT1" is preferred. Wikipedia



UTC: Is defined by atomic clock with corrections for leap seconds added manually.
UT1: Is defined by the Earth's rotation, hence is more susceptible for change and less easy to calculate than an atomic clock.


However the difference between the two is less than one second. It might matter for your $8000 \frac{\text{m}}{\text{s}}$ satellite or for your $300 \frac{\text{m}}{\text{s}}$ aircraft, but otherwise your can consider them equal for informal applications.