Your blood pressure is high and that might be causing giddiness to you.
Your neck pain
may be due to cervical spondylitis
and this in itself can contribute towards your giddiness.
Gastritis doesnt cause giddiness.
For gastritis you may have to take medicines for 6 weeks.
Take the following measures to control your high blood pressure
Restrict your calories
Shed your weight.
Stop smoking and moderate your alcohol intake
Reduce your salt intake
Avoid junk foods and colas
Consume fresh fruits and green vegetables
Do regular exercises
and deep breathing exercises.
You have to change your lifestyle to be healthy.