It looks like 2005 was a transition year between the 4L80E and Allison 1000 with 6.0 engine. Sounds like 4L80E was typical, but easily upgraded to Allison at time of ordering, but also depending on packages and equipment.
If it has an external filter it is the Allison, otherwise it is 4L80E version.
The 2005 Chevrolet Impala has a 4-speed automatic.
The 2005 Chevrolet Corvette has a 6-speed manual.
The 2005 Chevrolet Malibu has a 4-speed automatic.
The 2005 Chevrolet Express has a 4-speed automatic.
The 2005 Chevrolet Suburban has a 4-speed automatic.
The 2005 Chevrolet Colorado has a 5-speed manual.
The 2005 Chevrolet Tahoe has a 4-speed automatic.
The 2005 Chevrolet Equinox has a 5-speed automatic.
4L60E / 4L65E transmission
It should have a 6 speed Transmission.
The 2005 Chevrolet Express-Cargo has a 4-speed automatic.
The 2005 Chevrolet Silverado-1500 has a 5-speed manual.