Greetings, welcome to the forums.
Photos of the site would have helped, but on any significant slope, loose gravel would need to be held in place somehow. Trenches, various types of frames (wood, stone, metal, etc.) or binders such as concrete or mortar could all keep the pebbles in place, but all these methods would work well without pebbles at all! The sign can lay flat against the surface of the slope, or it can be raised and propped up to a higher angle. For example, the upright, vertical letters of the famous 'Hollywood' sign.
You never mentioned how permanent or temporary, these letters would be, but if you intend them to remain in place indefinitely, sturdy structural materials will be required.
Of course, there many more options for temporary signage, involving painted tarps, poster boards, and the like.