I have a good quality garden hose nozzle gun that I would like to bring to LOS.

Are the sizes universal or will my nozzle or US hose not fit with thai faucet and hoses?

Bring an assortment of male and female hose ends, some SS hose clamps with you. They will match up one way or another. Don't count on finding what you need easily in Thailand. I keep a bunch of everything in 1/2 and 5/8 brass, plastic, ends, menders Ys etc. Especially hard to find are the female ends that have threads that turn independently from the hose.

There's plenty of garden and hose options available here - so you can probably fit something from the states - but why? The main diff is that the states typically have screw stuff and here it's either clamp or, what I have, the snap on/off fittings (not sure what you call those). FYI: the normal Thai standard for a hose fitting is to wrap a piece of wire around the end of the hose to attach to whatever.

The other thing is water pressure. If you need pressure in the "up country" you better also have your own pump.

True Value Hardware is in Thailand and has a decent assortment of this stuff.

I predict your gardner or wife will take your stuff off the hose and go 'local'.