Big Re-Edit
I went back and reread the OP. I didn’t completely follow the setup, as a lot of others also did not do. Rather than elongate the thread I will just edit to add to my post. Perhaps others will do the same.
So here’s a more complete description:
Gun: DA revolver, .357 Magnum, Ruger GP100, Stainless, 6 inch
Note: This is in general, failures can happen to anything (including Glocks).
A revolver is more reliable because there is no failure to feed, no failure to eject, no jam. A misfire requires merely another pull on the trigger.
Ammo: 150 rounds .357 Magnum, 180 gr., JHP (this is about 5 lb)
I would not take any .38 Special because I don’t want any limits of power on the limited amount of ammo I can take. A 357 will do what a 38 Spec will do, but not vice versa.
JHP will handle most needs and because the gun is for emergency only, in my scenario, there will probably not be an opportunity to reload for the situation.
Sights: plain with laser added. Laser batteries last a long time if used sparingly and don’t add much bulk or weight, so no biggie when the batteries die. No optics, these are for long distance and I’ll trap or snare for food.
Holster: Cross draw with flap because cross draw puts the gun into a more comfortable position for me and is a faster draw for me. The flap is for protection from trash.
A lanyard? No. A lanyard will only get into my way.
Additional:
Maintenance materials for the revolver
Zip-Lock bags with canvas cover bags for revolver when not carrying and for the spare ammunition
The remainder of what I could carry would be basic survival gear for fishing, trapping, cutting tools, fire starter, medical supplies, shelter, etc. Especially important is plenty of socks, a spare pair of shoes, and a big jug of Avon SSS for mosquito repellant.
And a very small can of deodorant and shaving gear in case the OP forgets he already put someone here and he puts a woman on the other side of the island. (And soap to get rid of the SSS smell in a hurry.)