Gary Hymel, top aide to House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) for the last 8 1/2 years, is quitting to become a high-paid lobbyist here.

His departure will fuel talk that O'Neill, who has sadly watched many of his troops defect from the New Deal liberalism he personifies, many not run again.

O'Neill's reaction when Hymel told him his plans to put out firm word that he plans to attend the 1984 Democratic National Convention as speaker.

Hymel, 48, a former newspaper reporter in New Orleans, came here 16 years ago to be administrative assistant to majority whip and later majority leader Hale Boggs (D-La.). After Boggs disappeared in an airplane over Alaska during the 1972 campaign, Hymel went to work for the then-whip O'Neill, helped him nail down the majority leader post and has manned the desk outside the speaker's office since O'Neill moved there four years ago.

Hymel is in charge of the speaker's appointments, legislative scheduling, complaints from members and most other chores except for home district affairs and policy on difficult legislation. When O'Neill is out of town he keeps in touch through Hymel.

Hymel has turned down several offers of jobs with salaries in six figures. The first came more than four years ago, when the outgoing Republican White House congressional liaison team tried to make him part of their new lobbying firm. But Hymel wanted to work for a speaker.

Now, after four years as the speaker's administrative assistant, he will join Gray & Co., a public relations firm recently formed by Robert Gray, co-chairman of the Reagan inaugural and Cabinet secretary under Dwight D. Eisenhower. Hymel will give a Democratic touch to the company.

Hymel has told friends he decided to leave on the day last winter when Congress, in a fit of economy, voted not to give a pay raise this year to high-paid federal employes.