I'm looking for a gateway to pass from a 6LoWPAN over 802.15.4 network to 3G. Are there any on the market, or do I have to build one for myself based on Arduino or Raspberry Pi?
I think you will find modules without too much trouble, there have been chips around for over a year that I found (and there is a module on Kickstarter which looks like a simple small production run sales testing exercise).
A 6LoWPAN to 3G gateway is probably a bit of an esoteric use case - any you can find will be built-in to an end product, so it might be cheaper to assemble a unit from modules (or re-purpose an old phone maybe depending on the scenario).
Just to elaborate on the 'how' part, it seems to be common now for these radio interfaces to be integrated with a reasonably powerful MCU which handles the communication stack. Not just the link-layer, but some of the security aspects of the protocol too. So this would easily be able to interface to a GSM module through a UART, without needing a further processing device. Building the radio stand-alone would actually be harder than having it tightly integrated with a control processor. You should also be wary of trying to use the asic directly on your own board - using a module removes the challenge of RF layout and type approval.