It's called the zero-が and is before the object, in this case. Note that you don't need 私は for that sentence to be a fully valid sentence. It does not serve an actual grammatical function. The subject happens to be 私, and it is the unspoken subject. But every valid Japanese sentence which is not just an exclamation needs a subject, a "が"-marked subject, and if you can't see it you have a zero-が. For this sentence you would only use ”が” if you need to emphasize the subject, i.e. 私が牛肉を食べます - "it was I who..", or "I, in fact, .." - otherwise it's implicit, and not emphasized.

But the topic marker does not mark the subject. Any sentence with a topic can remove the topic, and the sentence is still a fully valid grammatical sentence.

For more about this, check out Cure Dolly's excellent videos about the zero-が on Youtube.

Edit: Various typos