Who’s playing whom？

其實你這句有兩個意思，因為在這裏的play可作”同某人比賽解”，又可作”扮演”解。
至於Who是作為疑問代名詞，是指”誰”、”甚麼人”
而whom也是用作疑問代名詞，作”甚麼人”、”誰”，不過是用作object，如，yahoo字典也有一例：
1. (用作疑問代名詞)誰,什麼人
I know who you are and whom you represent.
我知道你是誰並代表誰。
而playing whom並不是gerund phrase，而這句是這樣看：who (is playing) whom
who(疑問代名詞：主詞）＋is playing(現在進行式）＋whom（受詞）？
若這樣拆開後，整句的意思就非常明顯，是：
誰正跟誰比賽？（所以，也可譯作哪兩隊對賽）
或者　誰正扮演誰？（是指甚麼人正飾演甚麼角色），而且若你在網上搜尋，你會找到who should play whom或who would play whom等等，都是指著飾演來說。
真正的意思是甚麼意思，就要看前文後理了。

照字面解以上位朋友解釋得好好，但呢句其實係有一個口語化意思，就係一個人唔滿意另外一個人玩弄佢時，就會勞勞氣氣咁話：依家究意係邊個玩緊邊個？

whom 同who 係幾乎一樣，不過whom係用於object 而who係subject.
e.g. Sarah is giving a present to Emma 改為問題會變成
Who is giving a present to Emma? 答: Sarah
To whom is Sarah giving a present? 答: Emma

或者同黎連接
I'm going to meet Amy who goes to my school. 因為 I'm going to meet Amy. Amy goes to my school.
I'm going to meet Amy whom I don't like. 因為 I'm going to meet Amy. I don't like Amy.

Hope that has helped.