。
I'm guessing he's going to win the game.
Literal
I [topic-は] he [subject-が] game [at-に] win [nominalizer-の]-[is-not-ではないか] [quotative-と] guessing-[state-ている].
~のではないか ('might X / perhaps X,' literally 'is it not the case that X') is a tentative conjecture expression. 推測する ('to guess, conjecture, infer'). 試合に勝つ ('win the game') — 勝つ takes its opponent/match with に. A hedged prediction.