‘Sneezing uncontrollably‘ is a verb phrase.
Verbs are simply known as the ‘action’ words – may it be mental, physical or mechanical. When verbs are paired with auxiliaries (helping verbs), they are known as verb phrase. These helping verbs always go first before the actual verb. We consider it a phrase instead of ‘sneezing’ because of the comma, it emphasized the phrase. Sneezing if it had been a direct object or subject could be gerund, but unfortunately, not on this sentence.