So, Mr. GSH (G-Shock Highfashion) did a disassembly of a GST-S300 few months back. It's the first time I saw it last night. Very interesting stuff, especially to find out that the watch can be opened from front and back, and uses 2 large o-rings for water resistance.
It may freak some people out, as in: OMG, water can get in from the front also! But I say if it's done right, nothing to worry about. Not all GST models are this way, as mentioned by GSH in the video. The only thing is, if you have the watch for a long long time, you may need to change both o-rings. But the GST-S300 is a relatively new model, so you don't need to worry about that for a long while.
One benefit for opening from the front is that it's easier for those consider adding more lume to the watch face. I also wonder, are the higher end MTG and MRG models also designed this way with capability of opening it up from the front?