Hard to say what is best without knowing what type of model you are looking for.
Does your gk need GKRCS, Proxy or direct?
Does your gw have a billing interface?
If you gw does not have a billing interface then you probably will need a gk that provides gkrcs since in a direct mode, ARQ/ACF and DRQ are not a truly accurate method of billing.
If you have cisco gws, just get the cisco gk and get one of their partner based billing apps. If you choose mind, portal or digiquant you can leverage their pre/post paid card package to offer calling cards.