The attribute draw:gamma (section 20.128, ODF1.2 part 1) has no description, how to calculate the output color. Such description is needed, because there exists no standard for "gamma". For example in some situations ^value and in other situations ^(1/value) is used.
The .png format has a chunk gAMA, which holds a gamma value. http://www.w3.org/TR/PNG/#3gamma. And in that specification you get a detailed description, how this value is to be used for displaying a .png picture. http://www.w3.org/TR/PNG/#13Decoder-gamma-handling.
ODF should provide a description too. The mail https://www.oasis-open.org/apps/org/workgroup/office/email/archives/201304/msg00030.html has an attachment, which describes how Apache OpenOffice calculates the output color. I don't know how other application do it.
The specification needs in addition a rule for the order in which the color and tone adjustments are applied, see other subtask.
The value of draw:gamma should be restricted to positive values, because negative values and zero make no sense.