GFR tests kidney function. It is very important as we age it declines, it can also tell a doc if you have kidney damage from, say hypertension. Hypertension is very bad for kidneys because it becomes a vicious circle.
Your kidneys help maintain your blood pressure by releasing hormones like renin, aldosterone, and angiotensin. These help balance your fluids and control blood pressure, how much fluid your system keeps or releases, etc.

Exercise and weight control should also be on the doctor's order when trying to control your BP and your BP of 210/100 is dangerous. It can still damage both your kidneys and eyesight at 145/90 in only a short time.

The 100-90 diastolic indicates either your arteries are inflexible and out of shape because you haven't kept in good shape by fast walking and weight lifting, haven't let yourself get overweight; or you have too much fluid onboard due to kidney or lung issues, or you are drinking alcohol too much. Your lungs also help keep your BP in check, via interaction with your kidneys, so it could be a lung issue - but that isn't as important as having a kidney issue in blood pressure.

The systolic of 210 or 145 can indicate too much stress (adrenaline in your system), that you have been exerting yourself via activity or smoking.

Respectfully, your BP still needs to come down to avoid damage to specifically your eyes, kidneys, and capillaries (small blood vessels); an indicator of high BP is blood shot eyes as well as pounding in your ears and increases in tinnitus.

DASH diet and exercise can save your life.