Was Saul's conception and gestation part of God's plan?

Yes, Saul's conception and gestation were part of God's preordained plan. God had Saul on His mind in eternity past. At the appropriate time his mother conceived, and God formed him in the womb and brought him to birth, calling him by grace.

Les teaches that Saul of Tarsus was already on God's mind in eternity past. He points to Paul's words that God separated him from his mother's womb, explaining that at the appropriate time Saul's mother conceived, and the baby in her womb was all part of God's preordained plan. It was God who formed him in the womb and brought him to birth, and God called him by His grace—not because Saul deserved it, but by grace. This fits Paul's background as a persecutor whom God saw fit to save.

Galatians 1:15

But when it pleased God, who separated me from my mother’s womb, and called me by his grace,

When was Saul of Tarsus already on the mind of God?

Saul of Tarsus was already on God's mind in eternity past. Scripture indicates God separated him from his mother's womb and preordained the timing of his conception, formation, birth, and calling by grace. His salvation was not earned but according to God's purpose and grace.