The GC Performance Manager tracks STS, LTS and SB. (short and long term stress, stress balance).

TSS is trademarked, so GC uses BikeScore to calculate a ride's stress. BikeScore is calculated in a different way but comes up with very similar results. STS, LTS and SB are calculated from BS using the same algorithms that WKO uses to creata CTL, ATL and whatever the stress balance acronym in WKO is, from TSS.

At least for me, LTS and CTL from WKO are very close, like within a couple percent. That's probably closer than the accuracy of TSS or BS in representing stress.
I contributed the Performance Manager to GC last year.

FIT file support is in the developer builds (works great, I've been using it since I got a Garmin 500). It'll be in 1.4, which should be available soon.
If you can't build source yourself, read the mailing list- there are developer builds of proto-1.4 that are available for download.

I find that GC 1.3 is as good as WKO 2.2. 1.4 should be a bit better. I have not tried WKO 3.0.