Have you any old CDs you can hang up in nearby tree branches? They will flash in the slightest breeze and may deter them. We also had some success tying a metal coathanger to the top of a broomstick and arranging old waterproofs, with the hood up over the top of the broomstick, so that it looked like someone leaning on the gate. Whenever I went past, which was several times a day, I'd rearrange the sleeves or move the whole thing, so that they didn't become accustomed to it.