Significance of gl/gG rating on circuit breaker
This should be covered by IEC 60269.
gG and gL is the same thing where gL is an older definintion. 

Quote From Wikipedia article about IEC 60269


  The application cateogory is a two-digit code.
      The first letter is a if the fuse is for short-circuit protection only; an associated device must provide overload protection.
      The first letter is g if the fuse is intended to operate even with currents as low as those that cause it to blow in one hour. These are considered general-purpose fuses for protection of wires.
  
  The second letter indicates the type of equipment or system to be protected:


D – North American time-delay fuses for motor circuits, UL 248 fuses
G – General purpose protection of wires and cables
M – Motors
N – Conductors sized to North American practice, UL 248 fuses
PV – Solar photovoltaic arrays as per 60269-6
R, S – Rectifiers or semiconductors as per 60269-5
Tr – Transformers[citation needed]



  Any fuses built to the IEC 60269 standard and carrying the same application category (for example, gG or aM) will have similar electrical characteristics, time-current characteristics, and power dissipation as any other, even if the fuses are made in the packages standardized to the earlier national standards.


Unquote.

The gG/gL 100A that is mentioned in the question should be the fuse current rating. The current that a fuse in series with the device can continously pass through without opening. 

According to the comment below, the device marked with gG/gL is a Motor Starter Protector (MSP). This should be put in series with a fuse that won't trip unless a short circuit fault occurs. Not even a stalled motor should trip it. Have a look at Rockwell Automations has a paper "193-2.10: A Guide to Understanding: Short-Circuit Protection" which deals with this. There is mentioned that the fuse should handle approx. 6 to 8 times the Full Load Current. This corresponds quite well to the MSP mentioned in the question.

Here is a picture of how the MSP and Fuse works together: