Explain? There seem to be a lot of problems mentioned here with them. Of
course if you don't have room for a header tank, fine.

They're nothing compared to problems with tanked systems.

Some problems with gravity fed, I'm sure there are more.

1. Hard to refill. Air locks abound.
2. Unlimited loss of water in the event of a leak.
3. Failure to detect leaks for long periods due to replenishment.
4. Pumping over, introduces new water and air, leading to corrosion.
5. "Debugging" circulation issues very difficult and can prevent high pump
speeds.
6. Incompatible with most modern boilers.
7. Requires pipework in loft, which may freeze and leak.
8. Venting arrangements may be unreliable due to long or convoluted vent
pipework runs.

Christian.