Yes, the sentence is grammatically correct. It expresses a deep and lasting sense of gratitude towards someone for sharing or reposting something, likely on social media. The speaker is addressing the person as "sir," which suggests a level of respect or formality. The closing "Thank you and God bless you" further emphasizes the speaker's appreciation and good wishes for the person.
Here are some ways to rephrase it:
1. "I am forever grateful