Mk II, not Advanced one. It's a cloak for Freighters, Small Transports (these allowed to Freelancer ID) and Gunboats. As for the space, I can't recall it correctly, might be around 150-200.
The Cloak that you need you require is a MkII Medium Cloaking Device. They are typically priced at around 250-350 Million SC. Cloaking devices use cargo space to determine what size of device you need. A MkI Small cloak can cater to vessels that have up to and including 120 maximum cargo space. The MkII Medium cloak can cater for ships with cargo holds between 121 and 900 (Which is where your gunboat lies) and the MkII Advanced Large Cloak caters for any ship with a cargo hold between 900 and 2199. Cloak batteries are a must and for these three require no additional cargo space to use. There is a fourth Cloaking Device and It's the MkIII. It's for transports mainly, or anything with a greater cargo space then 2200 and the cloaking batteries take up cargo space for this cloak alone.
The MkII cloak that you require takes up 200 cargo space to mount. More information can be found here. Just scroll down to the cloaking device section.
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