Preheat to 350 degrees. Heat cream, garlic, and thyme spring in a medium saucepan just until bubbles begging to form around edge of pan. Remove from heat and set aside to steep.
Melt 1 Tbsp butter in a medium skillet over medium heat. Add leeks season with salt and cook, stirring often, until tender (do not brown). 10-12 minutes. Transfer to bowl and set aside.
Butter a 3 qt gratin dish with remaining 1 Tbsp of butter. Layer 1/3 of potao slices and 1/3 celery root slices evenly over bottom of baking dish. Cover with 1/3 of the leeks, then 1/3 gruyere. Sprinkle with salt, pepper and tsps thyme leaves.Repeat layers twice more. Strain cream mixture into a medium pitchcher and pour over vegetables.
Set gratin dis on a large rimmed baking sheet and coer tightly with foil. Bake for 1 hour. Carefully remove foil: continue baking until top is ogled brown and sauce is bubbliing, 25-30 minutes. DO AHEAD: Can be made 2 hours ahead. Let stand at room reptant with foil and rewarm in a 300 degree oven until hot, about 20 minutes.
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