My deceased grandma wrote a Will leaving money to her grandchildren. Can my grandpa take everything because they were married?
No. If you are sure that she left a Will you should consult with a probate lawyer who can review your situation and explain your options. If your grandmother owned any property in her own right she had the right to leave it to her grandchildren. However, she could not make gifts of any property she owned jointly with her husband.
You should ask to see the Will. It is illegal to withhold a Will after the death of the testator in most jurisdictions. An attorney could send a letter to your grandfather advising him that her Will, if any, must be submitted for probate. If you can't afford an attorney then you can visit the probate court and ask to speak with a clerk about filing a motion to compel your grandfather to submit the Will to probate.