Do I have to disclose that I have a masters degree if I am applying to a second masters in an unrelated field
Yes, you should reveal that you already hold an MS in CS. Don't be overly concerned about the GPA, but be prepared to explain your priorities at the time that had something to do with your grades. Hiding things is generally bad practice and most such things can be checked and any time gaps will certainly be questioned.
But your overall application, including letters of reference, has to be sufficient to allow someone to predict that you will be a success.
The time gap is probably in your favor here as people understand that others can change. Make sure that your statement of purpose is forward looking and points to success and clear goals.