I assume you're talking about GS OS 5.x, which essentially requires a mass storage device. (IE, hard disk or whatever.) The aforementioned Floppy Emu is probably the most easily accessible option and can emulate either a 3.5" floppy drive or a SmartPort hard drive, so it's probably a good compromise option. The cons are basically:
#1: SmartPort is a fairly slow protocol for a hard disk compared to a slotted option, although this probably isn't a huge deal, and:
#2: The Floppy Emu limits your ability to daisy chain drives *and* apparently if you want to boot GS OS from a Floppy Emu in SmartPort mode it has to be the first (which for practical purposes means "only") device in the chain, so it could be awkward to use if you want to use physical floppy drives too.
If you just want to fool with GS OS, though, it's probably fine.
Your
best option is probably something like the CFFA or the Reactive Micro Microdrive IDE controller, but neither is in production right now and the CFFA in particular tends to sell for a mint on eBay. Another option, and probably the most "period correct" one, is to get the Apple SCSI controller and either a real external SCSI drive or something like the SCSI2SD, but the SCSI card likewise tends to get spendy.
Finally, an off the wall and nerdy choice is to get an Ethernet to Localtalk Appletalk bridge and boot your IIgs off the network using
A2Server on a Linux box, VM, or Raspberry Pi. That's what I've done, and
it's actually by far the cheapest option. Its performance is somewhat inferior even to the SmartPort option, but it does work.