The Geometric moving average calculates the geometric mean of the previous N bars of a time series. The simple moving average uses the arithmetic mean, which means that it is calculated by adding the time series' value of the N previous bars and then dividing the result with the lookback period. The geometric mean on the other hand is calculated by multiplying the time series' N previous values (multiplication is used instead of the addition) and then taking the N'th root product of the last result.
Works like a charm.