GT Pro has problems recording in 24bit using certain WDM drivers. Your best bet is to download the latest MME drivers (they might be listed as drivers for Win 98/ME) and delete the GP2AUDDX.INI file in GT Pro program folder.
Another option would be trying out ASIO4ALL. These drivers are a good solution for generic or OEM sound cards that have unreliable WDM drivers and/or nonexistent ASIO support. Although you will not have ASIO capability in GT Pro 2 (only GT Pro 3 and 4) they may show up as MME drivers with 24bit recording capability. Please call 1-888-CAKEWALK (617-423-9004) or go to the Cakewalk Store for more information on purchases and upgrades.