For most practical purposes G, the gravitational constant, is constant. However, g the strength of the Earth's attraction, your weight does vary in several ways, both how strong it is, and the direction in which it pulls. None of those changes is large enough for you to feel directly, but they are enough to affect olympic records in certain sports, and they do cause tides and the like.

The largest effect is probably that caused by the rotation of the Earth. You weigh quite a bit more at one of the poles than on the equator, (especially on Mount Kilimanjaro!) and the effect is increased by the fact that flattening of the poles brings you nearer to the Earth's centre of mass.This difference can be more than a third of one percent.

Another large effect is caused by nearby large masses, such as nearby mountains or high density minerals underground near the surface. Conversely, you weigh less on a balsa raft on the deep ocean, water not being as dense as rock.

Then there are gravities of the sun and moon. By the moving of their masses they create tidal forces. These are minute, but genuine and measurable by sensitive instruments. In fact, they are much weaker than you would guess from the tides you can see. Tides are as large as they are because of a slopping action caused by the sun and moon, rather than their lifting the water that high.

I hope that is what you wanted to know.

Cheers,

Jon