Without context, it's not really possible to tell whether your example is correct or not.
If you were labeling a photograph, for example, you could say: "This is Gerard and me at a party."
But if you were telling someone that you are at a party with Gerard, you would say: "Gerard and I are at a party."
Like Laurence has tried to show you above, removing the second person from the sentence is the best way to check your grammar. In my sentences, if I remove Gerard we have:
"This is me at a party." Correct. If I had said, "This is Gerard and I at a party," once Gerard was removed from the sentence we would be left with, "This is I at a party," which is clearly not correct.
"I am at a party." Correct. If I had said, "Gerard and me are at a party," once I removed Gerard from the sentence, we would be left with, "Me at a party," instead of "I'm at a party."
You can always check your sentences this way so that, regardless of context, you know which option to choose.