Both are "true" DOHC cam setups, F heads just run the second cam off of a scissor gear on the first. How both cams get turned isn't really a definition of whether something is a true DOHC head, heh.
The main difference between the F head and G head design is the valve angle, F heads generally allow for more fuel economy, etc, over G designs.
I don't think there was ever a 5S-GE, only the FE, and generally speaking, Toyota likes to appease the North American market with larger, more torquey motors, and saves the nicer engines for other countries, lol.
As far as available engines for the Celica in North America goes, it's as follows:
86: 2S-E (ST161)
87-89: 3S-FE replaces 2S-E (ST162)
86-89: 3S-GE (ST162)
88-89: 3S-GTE (ST165)
90-93: 4A-FE (AT180)
90-93: 5S-FE (ST184)
90-93: 3S-GTE (ST185)
Then of course the 7A-FE and 5S-FE for the #T20# series Celicas. Outside of North America the Celica had many more engine choices, much like other Toyota vehicles, in fact, the 4A-GE was used in the RWD Celicas for 83-85 in Europe, in fact for 84 models, even the 2S was used in the RWD Celica in Europe, along with the R and T series motors, and of course the 5M-E, 5M-GE, and 6M-GE in the Celica Supra and Celica XX.
It's all marketing.
The main difference between the F head and G head design is the valve angle, F heads generally allow for more fuel economy, etc, over G designs.
I don't think there was ever a 5S-GE, only the FE, and generally speaking, Toyota likes to appease the North American market with larger, more torquey motors, and saves the nicer engines for other countries, lol.
As far as available engines for the Celica in North America goes, it's as follows:
86: 2S-E (ST161)
87-89: 3S-FE replaces 2S-E (ST162)
86-89: 3S-GE (ST162)
88-89: 3S-GTE (ST165)
90-93: 4A-FE (AT180)
90-93: 5S-FE (ST184)
90-93: 3S-GTE (ST185)
Then of course the 7A-FE and 5S-FE for the #T20# series Celicas. Outside of North America the Celica had many more engine choices, much like other Toyota vehicles, in fact, the 4A-GE was used in the RWD Celicas for 83-85 in Europe, in fact for 84 models, even the 2S was used in the RWD Celica in Europe, along with the R and T series motors, and of course the 5M-E, 5M-GE, and 6M-GE in the Celica Supra and Celica XX.
It's all marketing.