Your example sentence:
Also, make sure that you travel with proper medical insurance so that if you are sick or injured while traveling you will be able to get treatment.
No, you don't need any past participles, passive-voice constructions or anything like that. The sentence is absolutely fine as it stands. You're just failing to fully grasp the semantics of it. The word treatment there is a noun (a mass noun, to be more precise) which is basically just a short form of medical treatment (medical care given to a patient for an illness or injury). And the verb to get is used to mean to receive. So, we could actually paraphrase your sentence in the following way and it would mean exactly the same thing:
Also, make sure that you travel with proper medical insurance so that if you are sick or injured while traveling you will be able to receive medical treatment.
If you want the word treat to be used as a past participle, then the full expression would be something like this: to get treated for something. Where something specifies what you want to be treated for. But I don't think it will work as well as the way the sentence is originally written:
Also, make sure that you travel with proper medical insurance so that if you are sick or injured while traveling you will be able to get treated for your injuries.
I hope things make sense to you now. If you still have any questions, please don't hesitate to ask.