How do i handle territorial aggressive dogs i'm watching (house sitting?)

Contact your friend and ask them how they'd like for you to handle the situation.

These are not your dogs, and the owners of the dogs would be best in giving you recommendations on how to handle their dogs (or how they want their dogs to be handled).

Take charge.. first make sure you have heavy duty gloves and put the food and water on other side by cracking open the baby gate and sliding through...

Tell dog if firm voice to sit and give a treat do this several times. Tell the dog sit. and reward.

If this is sheltie they aren't that big but a bite would hurt. Meanwhile you can follow the other advice BUT if you let it outside it may be hard to get back inside without a collar/leash.

Any friend that leaves a dog without instructions and without a way to contact them is no friend sorry. If friend has relatives or neighbors that know the dog you can enlist their help.

good luck

Feed it treats and be firm with your voice when it barks, and say stop it. Then do babytalk as you hold out a treat. If necessary, put the bowl of food inside the baby gate and leave and wait until you can get hold of the owner. Make sure the dog has a bowl of fresh water.

It would have been better to get to know the dog before you were left with it.