Translation:Please open the gate.
As far as i understand, it's more common to write it in all kana when it is a supporting verb (like in the example sentence) and 下さい when it's the "main" verb (お水を下さい). I am but a student of the language myself, would be good for a native or a more learned speaker to confirm/clarify :)
This is comparable to seeing the word "laser" written out as "light amplification by stimulated emission of radiation". It is somehimes interesting to learn the origin of a word, but there can be many reasons to write the word in a more distant form. As I understand it, the art of using hiragana to replace kanji in certain situations dates back to a nationalistic desire to seperate and be less like China.