If you already have a sample collected and know the sample size, what would you want to compute in G*Power? Also, what type of statistical analysis are you looking at?

I'm with Christian here that, having already obtained a sample size, that there is no meaningful power analysis that can be conducted. I coincidently remarked upon this in a discussion on CV this week, where obtaining a power estimate after data is collected is not a useful practice:

https://stats.stackexchange.com/questions/641748/statistical-power-of-my-study/641759#641759

If there is more to your question, you may want to add clarification.