Craig Stevens is still the suave, unflappable title character and he's still spending his nights at Mother's, so there's a comforting sense of continuity in "Gunn." Granted, the supporting cast is different...but there had been two Mothers already, and frankly I was never a big fan of Lola Albright as Pete's steady flame Edie Hart. The only jarring change for me was Ed Asner as Lt. Jacoby. I like Asner, and he's the right general type for a curmudgeonly character like Jacoby, but I missed Herschel Bernardi's unique screen presence. (Was he uninterested in reprising the role, or was it not offered to him?)
There are a few misguided attempts to give Gunn a self-effacing sense of humor, but that was never a part of the original mix and it comes off awkwardly. Other reviewers have noted that the script recycles elements of certain episodes of the TV series, and they're right, but it relies even more heavily on the denouement of Howard Browne's 1949 detective novel "Halo in Brass." I'll offer no spoilers in that regard, but it's obvious that either Blake Edwards or William Peter Blatty was familiar with Browne's book.
Some graphic violence, some kitsch, and a reassuringly levelheaded performance by Stevens. (Gorgeous Sherry Jackson makes the most of a relatively small role; her fans will find this film worth seeking out.) "Gunn" is nothing special, but it's a reasonably entertaining attempt to update a quintessentially '50s character for the Groovy Age.