Rent a storage unit !

Packs and sleeping bags are going to be okay in the garage if you keep them away from moisture and vermin. Plastic bins with secure lids work well for this. If humidity is a big concern use airtight bins and some kind of dessicant in the bins.

Airtight bins do not mean that you have to mortgage your home for Pelican (although they make super-nice gear). There's much less rugged and less expensive stuff out there that will still keep humidity out.