John Cassavetes confirms the serendipitous nature of the film medium: No prominent American filmmaker has a clumsier style, yet there's something distinctive about his very clumsiness.

"Gloria," opening today at area theaters, is his most diverting terrible movie in some time. A preposterous combination of B-grade gangster melodrama and neo-feminist self-assertion, "Gloria" stars Cassavetes' actress-wife, Gena Rowlands, as a resurgent gun moll who decimates the ranks of the New York mob in order to protect a little boy, the sole survivor of a mob-connected family executed in mass reprisal for the indiscretion of the father, an accountant who has also consorted with the feds.

If the initial killings seem inexplicably indiscriminate, so do Gloria's subsequent massacres. Every significant aspect of the movie is over-emphatic to the verge of self-parody, beginning with the faintly ludicrous moans of a blues singer intoning "ma . . . ma . . . ma . . . ma . . ." over the credit sequence, illustrated by bold watercolors of urban vistas by Romare Bearden. The live footage opens with aerial panoramas of Manhattan more elaborate than the ones that supplied "West Side Story" with a portentous kickoff. Meanwhile, Bill Conti is busy imposing the most agitated jazz score in a generation or so.

Whenever the pace flags, as it frequently does, Cassavetes throws in another chase sequence and Conit goes into another musical frenzy. "Gloria" begins perilously high from both the pictorial and musical standpoints; the opening is so grandiose that the subsequent material is bound to be anticlimactic whenever a lull occurs.

Typically, Cassavetes cultivates a hotbed of gaucherie, absurdity, naturalistic cliche, explosive violence, vociferous overacting and wrong-headed brainstorms. His movies often appear to be a melodramatic and conceptual shambles, and "Gloria" is no exception. When Buck Henry turns up as the doomed accountant for the mob, you can't quite believe your eyes and ears. It isn't easier to think of him as the husband of a fiery Puerto Rican, played by Julie Carmen or as paterfamilias to their teeming brood. Although the household is about to be besieged, everyone finds the time to miss cues and strike stiff poses, another reliable sign that Cassavetes was behind the camera (waaaay behind, one gathers).

After the torpedoes blitz the family, they seem to spend leisurely hours sorting through the carnage and wreckage of the apartment, searching for Henry's incriminating ledgers. These have been entrusted to his son, now down the hall in Gloria's apartment. Rowlands, looking like a cross between Wallace Beery in "Bad Bascomb" (she even points her weapon in the funny way Beery used to) and Shirley Booth in "Come Back Little Sheba," has professed to dislike kids (the victim's kids in particular), but she naturally softens up and wipes out numerous assassins in the boy's defense.

The rapport that ought to evolve between Gloria and her juvenile charge never quite makes it from the filmmaker's imagination onto the screen. Cassavetes has chosen an unimpressive kid, John Adames, who sounds like a Munchkin with a Puerto Rican accent. This indifferent juvenile is also sabotaged by Cassavetes' redundant tin ear, forcing him into speeches like, "I am the man! I am the man! I am the man! I am the man!" and "I love you. I love you, Gloria. I love you to death." I heard, I heard.

Although she's a devastating shot, Gloria apparently can't decide whether she's coming or going. She keeps vacating her apartment and taking temporary lodgings and talking of hiding in Pittsburgh without making any decisive progress. Evidently aware of the incessant doubling back and repetition of situations, Cassavetes gives her a wonderful explanatory line late in the movie: "Boy, they got everything covered! Who knows, maybe they got Pittsburgh connected, too!" Sure, why bother schlepping to Pittsburgh when a scene can be repeated right where the actors are standing?

Rowlands brings off one irresistibly funny bit before Cassavetes settles down to working Gloria's lethal marksmanship, Clint Eastwood-like snarling and James Cagney seething into the ground. Confronted by a carload of mobsters on the street outside her Grand Concourse apartment building, Gloria pulls her revolver, delivers a fusillade, surveys the damage and then briskly yells, "Taxi!" Only a guest appearance by Robert De Niro as the arriving cabbie could improve on this delightful interlude.