Originally Posted by
DmaxMaverick
Welcome aboard!
You shouldn't have to replace the gasket, and the pan shouldn't have been removed for service (Allison recommends pan removal and internal filter service only at overhaul). The gasket is reusable rubber, and should survive at least one R/R, typically several. If it was removed, it's likely to have been improperly tightened, over, under, or unevenly torqued. Before removing, inspect and compare the gasket distortion and bolt torque in the area of the leak, and areas it is not leaking. It may only need tightening. If it's overtight where it's leaking, it will likely need pan removal/repair, and perhaps a new gasket if it's damaged. If/when you drain the pan, it will take just short of 2 gallons ATF to fill it again.
Also check for leakage at the vent on top. If you have an overfill or overheat condition, it can leak some fluid (trickles down the case, and drips off the pan). It can also leak if you have a torque converter drain-down condition (converter drains fluid while sitting idle for a period of days, causing "overfill"). The drain-down is much less common in later models, compared to 2001-2003 models, but can happen.