Tipping a gun range instructor is not a common practice. The focus at a gun range is on safety and training, rather than tipping.
However, some gun ranges may have a tipping culture, and it is always a good idea to check with the specific range you are visiting to see if tipping is customary or expected.
If you are unsure about tipping, it is best to ask the range staff or instructor for their policy on tips.
Citations:
[1] https://www.herocentermn.org/hire-private-instructor
[2] https://shootersworld.com/indoor-shooting-range/range-rules-policies/
[3] https://www.njgunforums.com/forum/index.php
[4] https://www.reddit.com/r/NJGuns/comments/dru2mm/tipping_an_instructor/
[5] https://www.trapshooters.com/threads/tipping-a-shooting-instructor.585217/