As Pud said, the 147 and GT's gauge acts differently from the 156.
On the 156, normal gauge behaviour with a working thermostat is that temperature gets up to the 70 degree line or the 90 degree line (depending on road speed, e.g. airflow) within about 5 minutes (or maximum 10 minutes on an icy day) of driving from cold and then does not drop down below the 70 line no matter what you do. Fairly big fluctuations between 70 and 90 on the gauge are normal. At idle or slow speeds, the needle will exceed 90 by a hair and then the fans will kick in, bringing it down slightly below the 90 line again.
If the temperature takes a much longer time to get up to the 70 line or above, the thermostat is dead (failed open). If the temperature drops below the 70 line (easily going down to close to the first, 50 degree, line) at highway speed after having warmed up, the thermostat is dead.
As Pud said, on the 147 and GT the gauge is "software modulated" like VAG products, where it will get up to temperature and then show a solid 90 at all times unless there is a big drop or rise in temperature. I have never driven a 147 or GT extensively, so I can't say from personal experience, but I do think that if a GT gauge shows anything less than 90, it probably indicates a dead thermostat.
What you describe sounds exactly like what a 156 of mine was doing when its thermostat had failed open. Replace it.