We've seen two versions of the Gunnar. One comes with its own 1/2x20 UNF thread replacement adapter.
I would pair it with the #S9, .25/30 Sumo if you have the .25 cal Gunnar.
If you have the .22 cal Gunnar I would go with the .22/25 cal Sumo, #S61.
If you have the other version with end cap it needs a custom #A10 adapter. Let us know if you need this one and we will create an invoice for you.
Other version with end cap needs a custom #A10 adapter. 7.5mm of exposed thread on #A10 adapter needed.