In order to find the converse of a conditional, we need to interchange the hypothesis and the conclusion of the sentence.
So we have:
Hypothesis: "Ruth is going to buy Gruyere cheese"
Conclusion: "She is not throwing a party"
Interchanging these, we have:
Hypothesis: "She is not throwing a party"
Conclusion: "Ruth is going to buy Gruyere cheese"
So the new conditional is:
"If Ruth is not throwing a party, then she is going to buy Gruyere cheese"