Bollinger Bands are similar to moving average envelopes. The difference between Bollinger Bands and envelopes is envelopes are plotted at a fixed percentage above and below a moving average, whereas Bollinger Bands are plotted at standard deviation levels above and below a moving average.
The standard Bolling Bands (BOL 20-2) can be called like that : GT::Indicators::BOL->new()
If you need a non standard BOL : GT::Indicators::BOL->new([25, 2.5])