If one of the target creatures becomes an illegal target in response to Verdurous Gearhulk's triggered ability, the +1/+1 counters that would have been put on that creature are lost. They can't be put on another legal target.

You choose how the counters will be distributed as you put Verdurous Gearhulk's triggered ability onto the stack. Each target creature must be assigned at least one counter.

Verdurous Gearhulk can be the target of its own triggered ability.