info_gug_seismometer is a special trigger for use in conjunction with a gug. This entity will fire its targets if a gug uses its earthquake attack near enough to the seismometer. The health key controls how sensitive the seismometer is – it will only fire if the damage level caused by the quake at the origin of the seismometer exceeds this health value. A value of 1 will detect quakes 800 units away, and a value of 9 will only detect quakes at a distance of zero. Note that the seismometer itself does not take damage or die, it triggers on every quake that exceeds the threshold.
This can be used as a purely decorative feature, paired with nearby info_rubble entities to create falling debris during quake attacks. A more complex set-up might involve physical entities collapsing during a quake to open up the route through the map. Remember there is no guarantee that the gug will cause a quake before the player kills it, so you will need a backup for opening the route on these occasions. Triggering off the gug’s death, delayed to match the tremor as he collapses, may be a good option.