I noticed that a lot of folks seem to be using plain ROOT to look at replay results:
root gmp_1204.root
Even though that works, you'll get "No dictionary available for ..." errors if you do this and lose access to things like the event header etc. that the analyzer writes.
The correct way to open replay results is with the analyzer itself:
analyzer gmp_1204.root
The analyzer command line works just like the plain ROOT command line. You have access to all the ROOT commands (plus additional analyzer classes), so you can draw histograms, run ROOT scripts etc. exactly as you do with ROOT. Also, there is no performance loss in using the analyzer instead of ROOT. The analyzer is just a thin layer of additional classes directly on top of ROOT.