Did George Washington actually state that Thanksgiving be a "Thanksgiving to Almighty God for all His blessings", in a proclamation in October 3, 1789?
To be technical, no. But I believe that the answer to the spirit of your question is yes.
If you look at the loc.gov link below, you will see that the actual words that you have quoted are not to be found in the document that Washington issued. So technically, Washington did not state what you are saying.
But I think the broader issue you are referring to is whether Washington signed a document referring to blessings from God. If that is what you are asking, the answer is yes. In the proclamation, Washington repeatedly refers to God and God's blessings. The lines most similar to what you quote are probably where Washington says that Americans should
unite in rendering unto him our sincere and humble thanks... in general for all the great and various favors which he hath been pleased to confer upon us.
So, Washington does not say the exact words that you give, but he does say something essentially similar in meaning.