スイスにある世界経済フォーラムは、世界の国で男性と女性の差がどのくらいあるか調べています。

I am in doubt in:
あるか I can't' find informations anywhere about this か on ある at the middle of the phrase.
what means がどのくらい (and why do i use ga particle)

Thank you!

是什麼意思

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The world economic forum held in Switzerland is investigating how much difference men and women have around the world.

The Japanese word order of 差がどのくらいある is ‘the difference how much is.’ 差 is a subject in the Japanese sentence. が is a paticle of the subject 差.