Locate and identify the -ing form.
sneezing uncontrollably, the teacher had to leave the room.
verb phrase
participle
gerund

This is a participle. It cannot be a gerund, because gerunds have a nominal or adjectival meaning (for example, Swimming is fun - swimming would be a gerund). The -ing form itself is not a 'verb phrase', but 'sneezing uncontrollably' is. So if we are looking only at 'sneezing', the answer is 'participle'. 

Idon’t take candy from strangers

Ive seen this! its d : )