Acute Gingivitis: This is the most likely diagnosis due to the direct association between gingivitis and gum inflammation, which can lead to blister formation. Gingivitis is a common condition characterized by inflammation of the gums (gingiva), often resulting from poor oral hygiene and the accumulation of plaque.

Periodontal Abscess: This condition can present with localized pain, swelling, and sometimes blister-like formations on the gums, especially if there's an accumulation of pus. It's a possible diagnosis if the blister is accompanied by significant pain and swelling.
Oral Lichen Planus: Although less common, oral lichen planus can cause lesions on the mucous membranes inside the mouth, including the gums. These lesions can sometimes appear as blisters.
Mucocutaneous Disorders (e.g., Pemphigus, Pemphigoid): These autoimmune diseases can cause blistering lesions in the mouth, including on the gums, though they are less common than other causes.

Oral Cancer: Although rare, any unusual or persistent lesion in the mouth, including a gum blister, should prompt consideration of oral cancer. Early detection is crucial for effective treatment.
Necrotizing Ulcerative Gingivitis (NUG): This condition is characterized by severe inflammation and necrosis of the gum tissues, often accompanied by pain, bleeding, and sometimes blister-like formations. It requires prompt treatment to prevent further tissue destruction.

Erythema Multiforme: A rare skin condition that can also affect the mucous membranes, causing blisters and ulcers, including in the mouth.
Aphthous Ulcers: While more commonly found on the softer tissues inside the mouth, in rare cases, aphthous ulcers can appear on the gums, presenting as painful blisters or ulcers.
Gingival Cyst: A rare, benign cyst that can form on the gums, potentially appearing as a blister-like lesion.