When I try to run NUnit tests in Gallio I get this:
Detected a probable test framework assembly version mismatch. Referenced test frameworks: 'nunit.framework, Version=2.5.3.9345, Culture=neutral, PublicKeyToken=96d09a1eb7f44a77'. Supported test frameworks: 'nunit.framework, Version=2.5.0.0-2.5.2.65535', 'nunit.framework, Version=2.4.8.0-2.4.8.65535'.
I use Gallio v3.1 build 397.
It's not tested but it will probably work. Try editing the Gallio.NUnitAdapter.plugin file for v2.5 and specify the new version.
Edit: Actually, after seeing this question I just made the change in trunk. Assuming it just works then NUnit v2.5.3 should be supported in Gallio v3.2 build 385 or newer.