All (or certainly most) outdoor lights, for residential premises, use a transformer which plugs into an indoor socket, so you don't need an electrician.
You can buy gutter hooks, which are specially made for the job, at most of the shops which sell the lights. Wilkinsons sell them, in packs of 16, for �1.49 each or 3 for �3. (Wilkinsons also sell 160 clear icicle lights for �8, 180 snowing icicle lights for �12, 315 snowing icicle lights for �17 or 480 snowing icicle lights for �25).
Chris