You mean a great deal to me这句话从语法上来说对吗?

是对的,这里的a great deal,相当于much,或a lot.
这句话的意思是你对我来说很重要.
a great deal 和a lot 差不多,如果后面要接名词的话,都要先接of.他们也都可以和你给的句中一样单独使用.比如：Thanks a lot.Thanks a great deal.
但a lot of后面既可接可数名词,也可以接不可数名词.而 a great deal of后面只接不可数名词.
e.g He has a great deal of money.
He has a lot of money.
He has a lot of friends.