The main difference is efficiency.
Open fronted gas fires use the room air to burn which makes them less efficient.
Sealed, glass fronted gas fires don’t need any air from your home – they draw air from outside the house in one flue, and expel the combusted air outside through another flue.  Less energy is wasted and there’s no risk of any gas smell or exhaust in the home.
There are many different kinds on the market, the biggest difference between the inefficient and the highly efficient gas fires is the way they are flued and how they bring in (and take out) the air used for combustion.
This is known as a Direct Vent system and it can make a fire up to 95% efficient – compared to an open fronted fireplace at around 45%.
Learn more about glass-fronted fireplace efficiency here.