when player pays 1 to buff unit at valley of idols, can i react to it before the unit gets buffed (for example gust the unit)

Yes — you can Gust the unit in reaction to the Valley of Idols trigger, before the buff is applied.

Valley of Idols' "When a player plays a unit here, they may pay [1] to buff it" is a triggered ability that goes on the Chain. The player pays the [1] as part of finalizing that ability, then you get priority to respond with a Reaction (like Gust) while the Chain is Closed. Your Gust resolves first, and if it returns the unit to hand, the buff instruction has nothing to buff — the [1] was already paid and is not refunded. This matches the general buff ruling: you can react to any buff before it resolves (FAQ #7927; Chain/Reaction timing, Rule 354.1).