Can a Kelly kettle be used on a Gas Hob?

The kettle will boil on a gas hob but it will be significantly slower than if sticks etc. were used.
The following in this FAQ should be helpful regarding what can and cannot be used to boil the kettle.
What types of fuel can I use in my kettle?
Can an alcohol burner be used with the Kelly Kettle?
In short, if it burns you can use it. The secret to a fast boil is to get an inferno going in the Chimney.
If you drop the kettle onto a gas hob it would be advisable to partially cover the chimney top with something to trap more heat inside.
Sticks will always beat a gas hob as the gas never gets a full scale inferno going. :-)