When I did a gauge swap out on a mustang i owned awhile ago i used a pry tool that fit between the gauge and the needle to pop the needles off.
Caution though make sure you tape up any tool you use so you dont mar up the gauges.
Also make sure before you pull them off you mark where all the needles point with engine off and with engine at idle and warmed up after ten mins. Also try to have your tank full to make it easier for the fuel gauge and make it as accurate as possible.
Those where things i learned when I did it awhile ago. Hope that helps.