Who exactly were the celtic people and how did they differ from a people who were called the 'germani' by the romans? Also what's the difference between galic and celtic peoples?
Not to discourage any further answers but these will probably be right up your alley:
What caused the Celtic expansions across Europe, and then the later Germanic expansions? Were there any shared causes? by /u/Miles_Sine_Castrum
You'll also probably like the FAQ on Celtic/Gaelic cultures