If a unit is being moved away from battlefield by via charm, gust etc can I use ambush unit on the same battlefield?

Yes, you can play a unit with Ambush to that battlefield, even if another unit is being moved away from it by Charm or Gust.

Reasoning

According to Rule 822.1.b and FAQ #8743, Ambush allows you to play a unit as a [Reaction] to any battlefield where you already control units.

1. Valid Location: Even if a spell like Charm or Gust is targeting a unit at that battlefield, your other units remain at that battlefield while the spell is on the chain. Because you still control units there, that battlefield remains a valid location for you to play your Ambush unit.
2. Reaction Timing: Because Ambush grants the unit [Reaction] timing, you can play it while the game state is Closed (which is the case whenever a spell like Charm or Gust is on the chain).
3. Independence: The fact that a unit is being moved away does not invalidate the battlefield as a destination for your Ambush unit, as long as you still control at least one unit there at the moment you play the Ambush unit.

Sequence of Events

• Opponent's Action: Your opponent plays Gust (or Charm) targeting your unit at the battlefield. This is added to the chain.
• Your Reaction: You play your Ambush unit to that same battlefield. Because you still have other units there, this is a legal play.
• Chain Resolution (LIFO - Last In, First Out):
  • The Ambush unit finishes finalizing and enters the battlefield first.
  • Then, the opponent's spell (Gust or Charm) resolves. If it still has a valid target (the unit it originally targeted), it will move that unit.

Important Note: As long as you have at least one unit remaining at the battlefield after the spell resolves, you maintain control of that battlefield. If the unit being moved was your only unit, you would lose control of the battlefield in the cleanup step (Rule 187.4.c), but your Ambush unit will have already landed, ensuring you maintain presence and control.