well, I would assume that gta02-core will produce working gerber files that can be put to a pcb manufacturer and SMT factory for actual component placement.
However, it is still a 2G or 2.5G device, based on an aged SoC... many years behind the kind of hardware that proprietary competitors are shipping.
So yes, this way you can continue to design and build open hardware cellphones - but you can certainly not produce something that the typical user will get excited about.
Using a more recent SoC like the s3c64xx (intended for GTA03) or an OMAP3 or i.MX51 or whatever else you might imagine is one aspect - and it is the easy aspect. The hard part is the actual 3G/3.5G modem. Unless you go into quantities of at least hundreds of thousands, there is no business case for any modem chip vendor to help you. And then those chip vendors are not only secretive/proprietary with regard to their hardware and firmware, they also are very secretive on the external interface/protocol.
An all those electrical design issues don't even speak about issues such as mass production, QA, mechanical engineering, distribution/sales, etc.