I've always thought a Battlebarge more of a specialized Battleship. Though since we don't have rules for them, Grand Cruiser works well enough in this situation.

Thankfully we have Battlefleet Gothic to use as a rough guide for how things work regarding Astartes ships. We know they're generally faster and more maneuverable, and generally more advanced than ships usually are (I doubt you have people hauling guns in to position with chains, on an Astartes ship, but then again... maybe they do?).

Well as the crews of Astartes ships are a mixture of:

• servitors (who can be quite strong)
• chapter serfs (who are recruits that didnt meet the cut but didn't die, so have some astartes implants and training, so are quite strong)
• full blow marines

combined with more ancient and advanced shipbuilding techniques and materials it means you end up with much less labour-intensive vessel. I'd imagine gun crews in the dozens not hundreds, but far more armsmen, weaponsmiths and technomats for example.