In the event of a detected fire, a GFP system will rapidly release an inert gas into the room that will suppress and extinguish the fire without damaging the room's contents. The rapid introduction of the gas will result in a significant over-pressure within the room, albeit of a short duration. This could result in lay-in ceiling tiles being lifted from the grid if sufficient and suitable pressure relief devices have not been included as part of the GFPS installation.
You should consult with your client to see if hold-down clips (e.g. BP A7890) are required to minimise tile movement. The use of metal clip-in tiles is an ideal solution for these types of installation because the positive gas pressure cannot dislodge the tiles from their grid.