1) Bit by bit 2) In a gradual manner 3) Leisurely 4) Little by little 5) Step by step 6) Unhurriedly 1) Bit-by-bit 2) Inch by inch 3) Increasingly 4) Overtime 5) Piecemeal 6) Progressively 
• (adv.) In a gradual manner. • (adv.) In degree.
Grad'u·al·ly adverb 1. In a gradual manner. 2. In degree. [ Obsolete] « Human reason doth not only gradually , but specifically, differ from the fantastic reason of brutes.» Grew. 
[adv] - in a gradual manner
bit by bit adverb in a gradual manner; `the snake moved gradually toward its victim`