関ヶ原 why is there a little ke?

Question:

and why is it pronounced ga?

Answer:

Originally, ケ meant like “の” or “of “ , when it was used in geographical name in ancient archaic Japanese language,
関 means “Gate” : “ケ” means “of”, “原” means “field”. So 関ヶ原 means “Field of Gate”

It usually used like combination of “feature of the place” + “ケ” + “landform”
袖ヶ浦 Coast of sleeve, which is equivalent to 袖の浦　in modern Japanese language.
鳩ケ谷 Valley of dove、which is equivalent to 鳩の谷　in modern Japanese language
幡ヶ谷 Valley of flag、which is equivalent to 幡の谷　in modern Japanese language
霞ヶ浦 Coast of mist, which is equivalent to 霞の浦　in modern Japanese language
市ヶ谷 Valley of market　which is equivalent to 市の谷 in modern Japanese language……

ケ used to be transcribed 箇 in ancient language, and later simplified to ケ when it used to mean particle　の
箇 could be pronounced ke, ko, ka, ga….. so ケ could be also pronounced ga when it used to mean particle の

This is against the modern Japanese Grammar, but it’s been customary used since ancient time and not easily changed regarding some old usages and name of places.