Mr. Gundotra in a blog post on Thursday said he was leaving Google after eight years at the company, stating "now is the time for a new journey." He will be succeeded by a Google vice president of engineering, David Besbris, the company said.
The departure comes shortly after a reorganization of the unit that included the departures of several product managers, according to people familiar with the move.
Google+, which was launched in 2011, has never seriously challenged the lead established by other social networks such as Facebook Inc.
Like Facebook, Google+ provides a stream of content such as photos and status updates that its users post for their friends. Its original selling points include a way to create groups of contacts, or "circles," to share more specific types of information. Another is Hangouts, a popular messaging tool that enables group video chat.
Google+ also was a key part of a new system the company created to help users sign into various services, like Gmail and YouTube, using a single login and password. Previously, those services had different systems to manage user identities.
Just how popular Google+ is remains a matter of debate. Mr. Gundotra said last October that the site had 540 million monthly active users.
But some current and former Google employees say that figure distorts usage of the core social-network elements of the site. The company has acknowledged that its definition of active users includes people who clicked a red notification icon in their Gmail inbox, for instance.
Counting only people who read posts, Google+ had around seven million daily active users two years ago, a person familiar with the figure said. Facebook reported in its most recent quarter 802 million daily active users.
Other events have raised questions about Google's commitment to the service.
In recent weeks, employees of the unit have moved from one building in the middle of the company's campus in Mountain View, Calif.—where it is easy for Chief Executive Larry Page and other top executives to keep tabs on the operation's progress—to another building farther away, people familiar with the matter say.
Proximity to Mr. Page's office is regarded as a sign of a project's status at the company, one former senior Google executive said, though others note that employee moves are common because of Google's rapid growth.
The choice of Mr. Besbris, a top engineer at Google+ rather than a person known as a product visionary, was also noted by some people close to the company as a sign of an uncertain future.
A Google spokeswoman insisted Google+ is growing and remains important to the company.
"Today's announcement has no impact on our Google+ strategy—we have an incredibly talented team that will continue to build great user experiences across Google+, Hangouts and Photos," she said.
Externally, the connections between Google+ and other Google services have sometimes proven unpopular. Some YouTube users, for example, complained late last year when the video-streaming service began requiring them to have a Google+ account to leave comments next to video clips.
"You've ruined our site and called it integration," sang one user on a YouTube video.
People who follow the company say the unified login has brought benefits to the company.
"Google+ is nowhere near being a competitor to Facebook," said Danny Sullivan, founding editor of Search Engine Land. "But it is important that they have managed to build a common identity platform that Google can use across its properties," he said. "Google has always been a collection of [services] and G+ has become this glue that binds them all together."
Mr. Gundotra was a member of Mr. Page's so-called "L team," a general staff of top executives that includes Sundar Pichai, who runs the Android and Chrome operating systems among other areas and Sridhar Ramaswamy, who is head of ads and commerce.
A longtime employee at Microsoft Corp. MSFT +0.43% before arriving at Google, Mr. Gundotra is described by some former colleagues as a polarizing figure whose style didn't fit in well at Google. But he is credited with helping to push Mr. Page to devote significant resources in an effort to catch up to Facebook in social networking.
One key motivation has to do with gathering information. While Facebook knows facts about users such as their names, ages and genders, Google was largely guessing about its users based on clues like their Web-browsing habits. Such extra information is important when you consider that the two companies are competing for the attention of advertising executives, who look to target ad campaigns to more specific slices of the population.
In a blog post, Mr. Page thanked Mr. Gundotra for "a tremendous almost eight years at Google" and wished him luck on his future activities. "In the meantime we'll continue working hard to build great new experiences for the ever increasing number of Google+ fans," Mr. Page wrote.
Mr. Gundotra couldn't be reached for comment. His blog post didn't explain specific reasons for his departure, or what he intended to do next. His blog indicates he engaged in personal soul-searching after a relative died in a traffic accident.
"Since then I've thought a lot about how similar this is to our life's endeavors," he wrote. "We pour our heart and soul into our work and it becomes something we love and cherish. But even the challenges we work on today will one day become 'and thens' as we move on to the next."