Grand Theft Auto has no in-game completion percentage and no reward for doing everything: progress is a target score per chapter, and the game only counts the chapter's Kill Frenzies as "secrets". This checklist follows the community 100% definition (gta.wiki): in each of the six chapters, reach the target score, complete every mission and every Kill Frenzy.
Totals (166 objectives): 90 missions on PC, including the 2 chapter introduction missions (88 on PlayStation/Game Boy Color: Counthash and Itali are PC only), 70 Kill Frenzies and 6 score targets.
How the percentages work: since there is no official split, each chapter's weight is its share of the required objectives (Gangsta Bang 11.45%, Heist Almighty 12.65%, Mandarin Mayhem 18.67%, Tequila Slammer 17.47%, Bent Cop Blues 19.28%, Rasta Blasta 20.48%).
Every mission can be attempted only once per playthrough of a chapter: a failed job is gone until you replay the chapter. Cheats do not block completion.