I'm talking about a pair of boots. "You need to buy boots with good grip" or "You need to buy boots with a good grip."
You will find the expression both with and without the article a.
But the alternatives tend to be used in different contexts.
We would say that: The government has a (good) grip on things, meaning that it is in control of the situation. Here it is necessary to use the indefinite article.
But with reference to shoes, tyres and the like, the use of the article is optional.
These are shoes/boots/tyres/wheels with good grip or with a good grip.
You can suit yourself.