Vestibular Migraine: Given the symptoms of dizziness and nausea, and considering the patient's history of GERD which might suggest a predisposition to migraines or other vascular issues, vestibular migraine is a plausible diagnosis. It's a condition known to cause episodic vertigo, often accompanied by nausea and can be triggered by various factors including stress, certain foods, or even gastrointestinal issues like GERD.

Benign Paroxysmal Positional Vertigo (BPPV): This is a common cause of vertigo that is characterized by brief, intense episodes of vertigo triggered by specific head movements. The patient's dizziness could be related to BPPV, especially if the dizziness is positional.
Labyrinthitis: An inner ear disorder that can cause dizziness, nausea, and vomiting. It could be a consideration, especially if the patient has recently had a viral infection.
Medication Side Effects: Given the patient's history of hypertension and GERD, he may be on medications that could contribute to dizziness and nausea as side effects.

Stroke or Transient Ischemic Attack (TIA): Although less common in a 39-year-old, it's crucial not to miss a stroke or TIA, as timely intervention is critical. Dizziness and nausea can be symptoms of a stroke, especially if accompanied by other neurological deficits.
Cardiac Arrhythmias: Conditions like atrial fibrillation can cause dizziness due to reduced cardiac output. Given the patient's history of hypertension, which is a risk factor for cardiac arrhythmias, this is an important consideration.
Hypertensive Crisis: A sudden, severe increase in blood pressure can lead to symptoms including dizziness and nausea.

Multiple Sclerosis: This autoimmune disease can cause a wide range of neurological symptoms, including dizziness and nausea, although it would be less common as a primary presentation.
Acoustic Neuroma: A benign tumor on the nerve connecting the inner ear to the brain, which can cause vertigo, nausea, and hearing loss. It's rare but should be considered if other symptoms like hearing loss or tinnitus are present.
Meniere's Disease: A disorder of the inner ear that causes episodes of vertigo, ringing in the ears (tinnitus), hearing loss, and a feeling of fullness in the ear. It's less common and typically presents with a combination of these symptoms.