There isn't really enough info to go on.
Are you already having counselling to support you with managing the anxiety? If so, can you discuss it there? (and if not, is it an avenue you've already exhausted?)
Your stepmother could be a flipping nightmare, and you absolutely have the right (even duty) to protect your daughter from her - even if she was actually your mother.
Or she might be lovely and your anxiety isn't related to her and it is affecting your daughter.
We can't know from what you've said.
What is clear is that you don't find your father supportive, and despite SM being part of your childhood, you sound like you have no affection for her. Her treating your child as a grandchild (not as a substitute grandchild, just as a grandchild) is not unusual. My daughter has a stepgrandfather and the relationship is lovely. If my SDs have children I shall be very happy to be treated as family.
So I don't think a SM sending gifts is a sigh of something bad in itself. But you know her, not us.
Something makes you anxious about being around them... So it's OK not to be 
Just I would say, explore counselling options rather than leaving it as it is.