Your problem is not same but surely you'll get that problem too after this solved if you have no .NET3.5 Full (not Client Profile) installed as well as .NET4 Full.
As Yan Sklyarenko noticed the specific version of .NET is requested by installer config. For latest distribution:
<Condition Message='.NET Framework 4.0 must be installed prior to installation of Git Extensions.'>
Installed OR NETFRAMEWORK40FULL
</Condition>
And also you have to have .NET 3.5 installed because of WiX installer use it. You should use full distribution again as Client Profile don't contains MSBuild component which WiX depends on:
Search "msbuild" (19 hits in 1 files):
...gitextensions-gitextensions-09f0e22\bin\Wix35.msi (19 hits)
There are also some GitExtensions modules targeted on previous .NET versions distributed with .NET3.5 while .NET4 is distributed separately.
Even Yan's cheat won't help you to work around this dependencies. At least you can save some traffic using web installers (4.0, 3.5) which downloads only modules you don't have yet.