The physical implementation of modern GDI has only been possible since the late-90s, which is really tied to the widespread adoption of common-rail direct-injection diesel.
Prior to common rail, diesels used a distribution-pump or camshaft-actuated unit injectors. For everyone talking about how direct injection "isn't new" and Mercedes was using it ages ago, that's mechanical fuel injection and actually not too far off from pre-common-rail distribution-pump diesel. Saying the system that Mercedes used long ago is like today's systems is showing a fundamental misunderstanding of how both current and past systems work.
Anyway, the move from distribution pump to common rail in diesels was predicated by the introduction of a electronic fuel injector that could live in a very high pressure fuel environment and be controlled precisely enough for the application. These first ended up in diesels primarily because of emissions. While early electronic fuel injectors could fuel an engine, it couldn't do it very cleanly. Diesels, naturally having looser emission standards, could get way with being dirty. There was no way a GDI engine with that technology would meet even the minimum Tier 2 Bin 5 federal emission minimum of the era: Too much HC and too much NOX.
Following the diesel story come the introduction of piezo injectors. These are what brought the fine control for fueling strategies with the ability for multiple pulses and actuations in a single combustion event. This was the big enabler for GDI engines. Injectors were now capable of fueling an engine and meeting emissions with elaborate multiple-pulse injection strategies. Someone also mentioned power required to process and command such strategies, which is part of this story too, for both diesels and gas engines.
Injecting fuel directly into the combustion chamber was, as a concept, something that researchers knew for a very long time would provide significant benefits but the technology required to implement matured in stages. Diesels first moved from indirect-injection to direct-injection while still using distribution-pumps, then technology allowed the use of a common-rail and electronic injectors, then piezo injectors. An analog is to look at cylinder deactivation: GM did it in the early-80s and it failed because, while the concept was sound, the technology wasn't robust enough to implement. 40 years later, it's common.