The waiting list for General Motors Co.’s GM -5.05%▼ new GMC Hummer electric pickup truck and related SUV model recently topped 77,000 prospective buyers. Most are likely in for a long wait.

GM’s renovated Detroit factory, where about 700 workers build the Hummer, has been producing around a dozen of the trucks a day, people familiar with the matter said. That pace is unusually slow for a vehicle in production for more than six months, manufacturing consultants say, and trails rival offerings from Ford Motor Co. F -3.39%▼ and Rivian Automotive Inc. RIVN -2.02%▼

Hummer production at GM’s Factory Zero, which underwent a $2.2 billion overhaul to build electric trucks, is on schedule, a company spokesman said. The ramp up has been slower than normal in part because the truck was developed from scratch using a new electric-vehicle platform. GM wants to ensure quality as it introduces the new technology, he said.

Output will increase sharply in the second half of the year, he said, largely because GM by late summer expects to begin using its own battery cells from a new factory in Ohio that it built with its joint-venture partner, South Korea’s LG Energy Solution. The Hummers built so far have used outsourced LG cells.

“Our ability to satisfy that demand is only going to improve as we bring on vertical integration of battery cell production,” a GM spokesman said. “You can expect to see hundreds of deliveries grow to thousands later this year.”

He added that customers are willing to wait for the Hummer EV because of its unique features. The Hummer has a longer driving range and faster charging times than rivals and has other features that make it difficult to compare it to other EV trucks on the market, the GM spokesman said.

Investors are watching closely as GM, Ford F -3.39%▼ and others test the nascent market for electric pickups, which is expected to emerge as an important battleground as car makers move into electric vehicles. Pickup trucks have long been the top-selling and most profitable vehicles for Detroit’s auto makers..

Auto makers are pushing to get electrics to market while also grappling with a computer-chip shortage and other supply-chain constraints that have curbed vehicle output and sales, particularly on gas-powered cars.

Compared to the Hummer, Ford is making about 150 of its F-150 Lightning electric pickups per day on average at the company’s factory in nearby Dearborn, Mich., a spokesman said. The company began production of the new truck in April.

EV startup Rivian RIVN -2.02%▼ built about 2,500 of its new R1T pickup trucks in the first quarter, the company said. Rivian and GM both said they began production late last year.

GM hasn’t disclosed a sales target for the Hummer, which is priced from around $85,000 to $110,000 depending on the model. Lightning prices range from about $40,000 to more than $90,000; Rivian’s R1T is priced from around $80,000 to $95,000.

Because of the Hummer’s higher price, GM likely is planning significantly lower sales volumes than those competitors, said Sam Fiorani, a vice president at research firm AutoForecast Solutions. He expects GM to position its Silverado as the direct competitor to Ford’s Lightning.

EVs make up less than 2% of unit sales for GM and Ford, and the market for electric pickups is untested. Still, executives from the companies over the past few years have been touting their electric-vehicle strategies amid surging investor interest in Tesla Inc. TSLA -1.76%▼ and other EV players.

Recently, investors and Wall Street analysts have questioned whether GM—among the first of the traditional auto makers to outline big EV ambitions—has ceded a head start to some rivals. The electric version of the Chevy Silverado pickup, is scheduled to go into production next spring, about a year after the Lightning’s launch.

In an interview this month, GM Chief Executive Mary Barra said she doesn’t think GM has been given enough credit for the Hummer, which has drawn media praise and went on sale ahead of the F-150 Lightning.

“We have 70,000 reservations for the Hummer. We’re delivering on it,” she said. “Frankly it’s a little stunning to me that people want to discount the Hummer.”

GM on Friday is scheduled to report second-quarter sales for the Hummer and the rest of its U.S. lineup.