The Laws of the Game do not specify a maximum gradient for the field of play.
Reality Check: Few pitches (if any) are perfectly level in every place. Even artificial turf fields have their variances. It is unreasonable to expect that a game cannot be played on a graded pitch for that reason alone. There are, however, other considerations.
Safety: Football associations, confederations, boards, and leagues have long held that safety of all participants is of the utmost importance, and it is the referee's duty to ensure it as much as possible (though he is not liable, under the Laws of the Game). This means that if the referee determines the field of play to be too unsafe for the game to be played (or becomes unsafe during play), the game must be stopped (and possibly abandoned, or not started if determined before kickoff).
If the referee decides (in his sole discretion) that a pitch gradient is severe enough as to be dangerous to player (and officials), the game will not take place. If the referee decides to begin play, and the gradient later proves to be unsafe, the match will be abandoned.
Laws 8, 13, 14, 16, and 17: These laws require that a ball be placed at a certain location and be kicked. The phrase "kicked and moves" present in each of these laws indicate (and in some cases, outright state) that the ball must be stationary prior to the kick.
If the gradient of the pitch does not allow the ball to come to rest reasonably close to the spot of placement, then the field should not be used. It is not permitted to use a placeholder, to dig a hole, or to use an artificial implement to hold the ball in place under any circumstance.
Law 18 - Common Sense: This is not an actual Law of the Game, but officials apply it to every decision they make. This is good, because referees get a LOT of discretion in their decisions, and using common sense is preferable to not using any at all!
If a the gradient of the pitch is just plain stupid (like a 1m hill in the center circle, or a goal sitting on a cocked angle due to terrain), the sensible referee should refuse to start the match.