The renewable energy investment sent GPSC's shares up 4.36% in morning trading against a 0.4% decline on the benchmark index at 0500 GMT.
GPSC said the move increases its renewable energy capacity to 2,145 megawatts (MW) or 32% of the total capacity of 6,613 MW.
GPSC, a unit of state-owned energy giant PTT Pcl, considers India as a focus country for renewable energy expansion.
"The investment in this platform aligns with the company's growth strategy in the renewable energy business," GPSC chief executive Worawat Pitayasiri said in a statement.
Avaada has solar power capacity of 1,392 MW with another 2,352 MW under construction, which will begin commercial operations by 2022.
"This is a positive surprise," said Finansia Syrus Securities analyst, Suwat Sinsadok, adding that the acquisition cost was reasonable.
The deal comes days after another PTT unit, PTT Global Chemical Pcl, announced a $4.75 billion acquisition of German coating resins maker Allnex.
($1 = 32.6500 baht)
(Reporting by Chayut Setboonsarng; Editing by Ed Davies, Martin Petty)