I agree with others about GR5 being simply not very good amp sims. It has a lot of terrific FX in it, so if you don't care that much about the underlying amp tone or perhaps if you are not accustomed to what a real tube amp sounds and feels like, then GR5 might be great for its flexibility..perhaps some of its cleaner tones might be ok. The OP never really said what kind of guitar tone he is primarily needing, which could be a factor in recommending something.
I have the following, some of which are free and you should definitely try the free stuff, some of it is quite good:
GR5
Amplitube4 (including Hendrix, Fender2, Slash and other models)
Peavy Revalver
Vox JamVox
S-Gear
Thermonik (most classic models)
Recabinet 4 and 5
Anvil
Hybrit
Le456
LeCto
LeGion
LeXtac
LeCab
Nick Crow 7170
Nick Crow 8505
Nick Crow Tube Driver
Nick Crow Wagner Sharp
StudioDevil
Ignite TPA-1
Ignite TS-999
Ignite TSB-1
TSE-R47
TSE 808
Ignite Emissary
They all have some good things and bad things, its hard to pick a favorite but strangely my favorite is the Vox JamVox which which sounds almost exactly like my Vox Tonelab, which I still love to this day...especially for brown tones, its not particularly great at strong metal sounds, but many of the free ones listed above are.
Thermonik is very underrated also and very good, I want to do more with it, but it doesn't really have any FX compared to others
Amplitube4 sounds pretty darn good in some models, but i find it dreadful to play in terms of ear fatigue and feel.
Revalver has amazing customization options, including you can design your own amps down to the tube circuits. you can also insert 3rd party VST's into the signal chain inside it.
S-Gear is supposed to be great, but I just haven't gotten along well with it or gotten great tones out of it, so I don't know... Need to spend more time with it.
There are some others that I don't own, haven't used but worth mentioning
Positive Grid
Overloud TH3
Waves GTR3
Brainworx
Mercurial
TSE X50
Line6