Gandalf and Pingers Interaction

Hello sorry upfront if I don't phrase this question well. Still pretty new to magic and learning all the terms. But, I have a question about [[Gandalf the Grey]]. I don't quiet understand the untap ability he has as to when it can happen. Lets say I have [[Gandalf the Grey]] , and [[Thermo-Alchemist]] on the board, I tap Thermo to deal damage. I then cast a spell untapping him. Can i then tap him again before I declare I'm using Gandalf's ability to untap him thus giving me three activations of Thermo?

Again sorry if that was poorly described. Thank you in advance