I asked the question today and Tria replied:
A treatment gel is not recommended as it may penetrate and block the laser light & prevent it from working effectively.
In fact, we recommend customers use a cleanser to rid their skin of all creams, moisturisers, sun block in fact any barrier that may prevent the laser from penetrating the skin. The Tria cleanser includes ‘ a gentle exfoliating lactic acid ‘ to help remove any dead cells. If customers experience a dryness when using the cleanser we recommend they discontinue use for a period of time or use the cleanser less frequently, that is every other day, every 3 days etc.
The serum does help soothe the skin after treatment. Our US team recommend leaving the serum in the fridge as the cooling effect can help reduce redness.