Gold soft solders very easily with standard Lead/Tin solder and flux.
The newer Tin/Silver soft solder will work as well.
Doesn't matter if its 24k or lesser karat gold like 14, 18 etc.
For this you don't need a specialty 'gold solder' as the solder joint is near invisible as a butt type joint with the steel sight.

It's just like soldering brass.
Clean the surfaces, make for a good fit.
Tin one or both surfaces. Flux both. Assemble and heat to flow the solder.
Don't burn the flux,,don't over heat,,all the same rules.
On small stuff like a sight, an elect soldering gun works very nicely.

If you have a wider face sight blade you can place a gold dot into that surface.
Either with real gold or common brass. Silver works too. Even copper works nice.

Make the bead of a diameter that reaches to the sides and the top edge of the blade.
The center of that circle needs a small hole drilled into the face of the blade.

The bead material then needs to be turned to produce a short peg on it to fit into that hole.
The bead then soft soldered into/onto the blade.
Excess is cut off w/a jewelers saw and the bead is left standing high of the steel sight blade.

The Bead can be left standing proud with sharp edges, or be rounded slightly.
Sometimes the bead is taken right down flush to the blade surface. Some even are lightly countersunk.
All sorts of ways to make them

If you can inlay gold, silver, copper or brass, the above style bead can be inlayed instead of soldered into place.
It takes some skill but is a common method. Open a cavity on the face of the blade, then undercut it with the same punch. Just don't go outside the diameter of the bead to be overlayed or
the undercutting work will show.

Many of the domed bead sight styles are made that way,,or were. The final 'setting' of the bead is done with a cupped punch, highly polished
of the correct size and depth you want for the finished bead.
You make those setting punches if you do ths work, or at least that's how it used to be.
Upon setting the bead, it shapes it, sets it down to the blade all around w/the edges perfectly tight.The high polish finish of the punch leaves the bead w/the same polished luster.
(Engrave your initial, a star, a circle, etc inside the punch and it'll be on the face of the bead...)

The small pin head bead sight like the one in the pic in the link I usually use a piece of brass or copper rod of close dia. Clean the seat in the blade to accept it and soft solder it in place. Leave the rod long so it's easy to work with.
Sometimes an older sight will have a hole in the face of the round portion for you from an old bead insert. You can make use of that for extra hold. Carefully file a small peg on the rod so it inserts itself into that cavity.
Clean everything out well and soft solder in place.

When cooled off, cut the excess rod length off. Then carefully file the bead lengthwise to proper dia with the sight as a guide..

Done with care, soft soldered attachments even this small are very rugged and will give good service.