When resolving milestones, if a player passes multiple milestones in one turn, the order is as follows:

• Milestones are resolved between turns, after the current player has finished playing free actions.
• If multiple players have reached milestones, they resolve them in order, starting with the player whose turn has just ended, and proceeding clockwise.
• The neutral milestone belongs to nonplayers, and it is always resolved last.

This means:

• If a player passes both a gold and a neutral milestone in the same turn, the gold milestone is resolved first, followed by the neutral milestone.

Source: Page 18
"Milestones are resolved between turns. If multiple players have reached milestones, they resolve them in order, starting with the player whose turn has just ended, and proceeding clockwise. ... The neutral milestone belongs to nonplayers, and it is always resolved last."

Source: Core Rulebook (pp. 18, 19, 21, 23, 28)