A globe-shaped object is hard to create a stencil for because it curves in two directions. To have access to the painting surface, the stencil shape would be a cylinder.
If you make the stencil from tape, it can be difficult to position it because only the very edge of the tape can form a cylinder; if any other part of the tape grabs the surface while you're positioning it, the tape won't form a clean, uniform circle around the object. Once the edge is pefectly in place, the rest of the tape can by crumpled against the object. Tape also runs the risk of pulling paint off the surface if the paint isn't well adhered or the surface is weak.
Vehicle pinstriping tape is suggested in fred_dot_u's answer. That is stretchy so it can conform to a 3D surface. But the adhesive is designed for automotive paint, which is very tough stuff, adhered to a tough surface. You would want to experiment with it, or any tape, on painted plaster.
My suggestion is to use a rigid cylinder that isn't adhered to the object where that will work, or tacky putty. The tacky putty I'm referring to is a putty that sticks non-aggressively to a surface and is designed to not stain or damage the surface. There are various such products sold to stick light-weight objects, like photos, temporarily to painted walls, or to seal weatherproofing film to painted walls. Some of these putties can leave a stain if left in place for a long time, so you would want to test the product in an inconspicuous place (I haven't recently used any, so can't make a specific recommendation).
You can use the tacky putty in combination with a rigid cylinder to create a mask (position the cylinder, then press a band of putty against the cylinder edge to create the stencil edge, then remove the cylinder), but you could also use it with the kind of setup described in fred_dot_u's answer, where you would press a band of putty around the object, then use a knife in that setup to precisely cut it for the edge of the stencil, and peel off the putty in the painting area.
With a rigid cylinder or tape, use thick paint so it doesn't wick.
Cylinder approach details
Measure the diameter at the top and bottom of the area where you want a paint band. Make a cylinder of each diameter from sheet material with enough strength to support the weight of the piece, like cardstock. Ensure that you start with material that has a straight, clean edge.
If the paint band is around the equator, support the piece on the bottom cylinder, and position the top cylinder so the two cylinders create a stencil for the paint band. Note that the top and bottom cylinders don't need to be the same size, so the paint band doesn't need to be the same distance up and down from the equator.
If the band will be entirely north or south of the "equator", position the larger cylinder, and use its edge as a guide for applying the putty (or painter's tape). Remove the cylinder and then support the piece on the smaller cylinder so it acts as a stencil. The image below shows the putty/tape and cylinder at the top, but you can flip it upside-down so the cylinder acts like a stand.
Note in the diagram that when you need to use tape, the edge you want perfect is on a smaller diameter than the other edge of the tape, so it's almost impossible to lay down tape perfectly. Using putty solves this problem.
As Elmy notes in a comment, you need a method to ensure that the cylinders and object are accurated aligned horizontally, like using a level. You also need to be careful to keep the positioning secure so nothing shifts while you're painting.
Last resort masking
If you are concerned that any form of tape or putty might damage the gold, and don't want to risk trying to touch it up, here's a last resort method to safely mask. Wrap the piece tightly in plastic wrap. Then apply painters tape tightly in the area and vicinity of where the band will be, overlapping where the edge will be.
Measure the diameter after it's wrapped to make the cylinder. Use the cylinder to identify where the edge of the band will be, and cut the wrapping with a knife (if that creates a groove in the plaster, it will be filled with paint). Remove the wrapping from the side you will paint, and use the smaller cylinder to complete the stencil. The painters tape will peel off the plastic film to remove the mask.