Is there a word for grandparents(collectively)?
Unlike brothers; sisters (兄弟; 姐妹), Chinese don't have a collective word for 祖父母 (grandparents from father's side) and 外祖父母 (grandparents from mother's side)
If you had to write grandparents and didn't know if there are maternal or paternal grandparents, you can write  父母的父母 (the parents of one's parents), which encompasses one's all four grandparents
We do have a collective word 祖輩 for 'ancestors' and great grandparents are included in this term so you don't have to write 父母的父母的父母 for it
Edit:
祖父輩 (Grandfather's generation) encompasses the whole generation, which includes grandparents and granduncles