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'.