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* GameStop faces 'unsustainable' sales decline, shares slide

* Merck up after FDA approves therapy for rare lung condition

* Megacap growth stocks, chipmakers recover

* Futures up: Dow 0.37%, S&P 0.35%, Nasdaq 0.38%

March 27 (Reuters) -

Wall Street's main stock indexes were set to gain at open on Wednesday as chipmakers and growth stocks rebounded in light trading ahead of crucial economic data, commentary from the Federal Reserve's policymakers and a long holiday weekend.

The Dow and the S&P 500 recorded their third consecutive declines on Tuesday, as most megacaps were pressured and stocks struggled to maintain an upwards momentum.

Investors looked forward to comments from Fed Board Governor Christopher Waller, who is set to speak at the Economic Club of New York later in the day.

The Personal Consumption Expenditures Price Index (PCE), the Fed's preferred inflation gauge, is due on Good Friday, when the U.S. stock market will be closed.

An upside surprise to inflation could potentially dampen market enthusiasm around early rate cuts.

Also on tap this week is the final fourth-quarter GDP print, the University of Michigan's reading of consumer sentiment and weekly jobless claims data.

All three major U.S. stock indexes eye quarterly gains as an artificial intelligence-inspired rally helped Wall Street reach record highs recently and optimism about the Fed cutting borrowing costs later in the year added to gains.

Traders see a 70% chance the Fed will begin its easing cycle in June, according to the CME FedWatch tool.

"The market is just doing a little bit of consolidation... people are just wondering what's the next catalyst to drive stock prices," said Robert Pavlik, senior portfolio manager at Dakota Wealth Management.

"The focus on AI is just continuing to power interest in technology, and especially the semiconductors and the companies that make AI happen."

At 8:20 a.m. ET, Dow e-minis were up 145 points, or 0.37%, S&P 500 e-minis were up 18.25 points, or 0.35%, and Nasdaq 100 e-minis were up 70.25 points, or 0.38%.

Most megacap growth stocks rose in trading before the bell, with Tesla adding about 1% and AI-giant Nvidia advancing 0.5%.

Several chip stocks also gained, with Micron Technology inching up 0.5%, while Marvell Technology jumped 4.1%.

Moderna added 3.9% after the biotech firm said it had entered into an agreement with private equity firm Blackstone's life science investment platform for $750 million in funding to develop its mRNA flu vaccines.

Merck & Co gained 5.2% after the U.S. Food and Drug Administration approved its therapy for adults with high blood pressure due to constriction in lung arteries.

Trump Media & Technology Group jumped 13.2%, a day after its stellar Nasdaq debut.

Robinhood Markets climbed 7% after the online trading app launched a new credit card.

GameStop dropped 18.1% after the videogame retailer reported lower fourth-quarter revenue and said it had cut an unspecified number of jobs to reduce costs. (Reporting by Shashwat Chauhan and Bansari Mayur Kamdar in Bengaluru; Editing by Rashmi Aich and Pooja Desai)