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TL;DR: the old definition of the meter was based on the length of a string necessary to make a pendulum oscillate once each 2s. And since the oscillation of a pendulum depends on both gravity and π, by assigning that length the value of one unit, you end with gravity as π².

This gets easier to see if you actually include the units into the equation:

T = 2π√(l/g)   // the formula for pendulum oscillation
T² = 4π²l/g    // getting rid of square roots, bloody roots
g = 4π²l/T²    // same junk as above, just reorganised
g = 4π²l/(2s)² // they chose a pendulum with 2s oscillation
g = π²l/s²     // cancelling out 4 and 2²
g = π²(1m)/s²  // "we shall call that length ONE METRE!"
g = π²m/s²     // gravity is pi², and the units fit