In terms of PSTN gateways, Axon can work with many TAPI compliant voice boards (see the NCH website for more info) as well as networked FXO gateways like the Linksys/Sipura/Cisco SPA3000.
Skype calls can be handled through NCH's uplink application (costs extra). In my personal opinion however, it is not as stable as it could be and it has not been updated in sometime (i.e. may not even work with recent skype client versions). There are alternative software packages available on the net that can turn Skype into an SIP gateway including the official Skype to SIP gateway service (there is however a fairly hefty licensing cost on a per line per month basis).
I don't excplicitly know of any gsm gateways that work with Axon, but if it has a normal analog RJ11 phone port on it, you can probably just use it like a regular landline connection. If not, there may be some compatibility issues.