Can a number of calories be measured in grams?

For example, 200 calories is x number of grams?

I found some app where you enter what you’ve eaten and it calculates, in detail, how many calories did you enter. So, if you enter your weight and target weight, it can calculate how much do you need to burn, or how much you’re allowed to eat today.
It said I can eat 284 calories today to in order to reach the target weight.

The thing is, I’m about to attend some gathering, I’ll be served food, and I have no idea how many calories will be in there.

How much, in grams, approximately, should I eat in order not to overeat?

Considering the meals in all gatherings here are more or less the same, I expect stuff like a soup, grilled meat (probably pork or lamb) and cake. I assume I should avoid sodas and sweets but what about the rest?