Unless you strip the paint entirely and clean up the metal and remove the rust it will still be there under the paint eating the metal away. The gates will just fall apart due to rust, probably in a lot less than 10 years. It will be time consuming but if you want to save the gate it is the only way. You say they only cost £20, but what you need to worry about is how much it will cost to replace them. That will be more than £20 .
Just painting them is useless, as I said the rust is still there under the paint rusting away the metal and it will not stop just because you keep putting more paint on top.