There is a shimano converting spacer on ebay, from an engineering perspective it’s suspect as shimano BB’s aren’t radial bearings, they’re ACB like your headset and rely on a little preload. But it seems to work for a lot of people. I suppose it might work with other aftermarket BB’s that do have radial bearings. But you’ll run int the problem I had below.
I wouldn’t recommend any aftermarket GXP BB, I’ve tried Hope, Gusset, Superstar and some others and none had a cir clip to hold the bearings in, so they just fell out after a while! Because GXP doesn’t squeeze the bearings in any way it needs something to hold them in the shell! Infact even SRAM don’t retain the plastic top hat ont he drive side well enough so that walks out and a spacer needs bodging to hold it in (old Race face X-type 24mm x-rings work well for this but any old 24mm o-rings or spacers should work, just add 1mm shims until it starts to bind then take one off).
I’d either get a new SRAM GXP BB (I tended to find they lasted long enough to be reasonable) or buy a shimano chainset.