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Since Blender 3.4.1, in Draw Mode of a GP Stroke, there's a checkbox Outline in the top menu bar under Stroke.

This amazing new feature creates not the center line of the Stroke, but instead creates the perimeter Line (inluding varying thickness from pen tablet):

So, this might be a good start. Depending on your Stroke geometry, you might want to use a Simplify Modifier on it. If you have overlapping strokes, try the Cutter in Draw Mode. For cleanup, convert Stroke to Polyline, convert again to Mesh and Merge at Center in Edit Mode:

After cleanup, select all and hit Alt+F to fill. This can fail at the caps of the stroke, depending on geometry. Just fill the failed Areas with Ngons (select that area's border, then just hit F).

Finally, add a Solidify Modifier:

Since Blender 3.4.1, in Draw Mode of a GP Stroke, there's a checkbox Outline in the top menu bar under Stroke.

This amazing new feature creates not the center line of the Stroke, but instead creates the perimeter Line (inluding varying thickness from pen tablet):

So, this might be a good start. Depending on your Stroke geometry, you might want to use a Simplify Modifier on it. If you have overlapping strokes, try the Cutter in Draw Mode. For cleanup, convert Stroke to Polyline, convert again to Mesh and Merge at Center in Edit Mode:

After cleanup, select all and hit Alt+F to fill.

This can fail at the caps of the stroke, depending on geometry. Just fill the failed areas with Ngons (select that area's border, then just hit F).

Finally, add a Solidify Modifier:

Since Blender 3.4.1, in Draw Mode of a GP Stroke, there's a checkbox Outline in the top menu bar under Stroke.

This amazing new feature creates not the center line of the Stroke, but instead creates the perimeter Line (inluding varying thickness from pen tablet):

So, this might be a good start. Depending on your Stroke geometry, you might want to use a Simplify Modifier on it. If you have overlapping strokes, try the Cutter in Draw Mode, then merge by distance in Edit Mode:

Next, you could convert the GP Stroke into a Poly Curve and use a Screw Modifier (with Angle = 0° and Screw = x) on it, so until now, it's still semi-nondestructive:

At last, you would need to convert it to a mesh, which automatically applies the Screw Modifier (make a backup duplicate first!). Then, you can enter the Edit Mode, select the Borders with Alt+Leftclick

and hit F to fill them. When you get weird shading, it's because the original curve had smooth shading. Try Shade Flat.

Keep in mind that the Fill command creates NGONS - it's not very good Quad Topology. But I guess it still gets the job done.

Since Blender 3.4.1, in Draw Mode of a GP Stroke, there's a checkbox Outline in the top menu bar under Stroke.

This amazing new feature creates not the center line of the Stroke, but instead creates the perimeter Line (inluding varying thickness from pen tablet):

So, this might be a good start. Depending on your Stroke geometry, you might want to use a Simplify Modifier on it. If you have overlapping strokes, try the Cutter in Draw Mode. For cleanup, convert Stroke to Polyline, convert again to Mesh and Merge at Center in Edit Mode:

After cleanup, select all and hit Alt+F to fill. This can fail at the caps of the stroke, depending on geometry. Just fill the failed Areas with Ngons (select that area's border, then just hit F).

Finally, add a Solidify Modifier:

Since Blender 3.4.1, in Draw Mode of a GP Stroke, there's a checkbox Outline in the top menu bar under Stroke.

This amazing new feature creates not the center line of the Stroke, but instead creates the perimeter Line (inluding varying thickness from pen tablet):

So, this might be a good start. Depending on your Stroke geometry, you might want to use a Simplify Modifier on it. Next, you could convert the GP Stroke into a Poly Curve and use a Screw Modifier (with Angle = 0° and Screw = x) on it, so until now, it's still semi-nondestructive:

At last, you would need to convert it to a mesh, which automatically applies the Screw Modifier (make a backup duplicate first!). Then, you can enter the Edit Mode, select the Borders with Alt+Leftclick

and hit F to fill them. When you get weird shading, it's because the original curve had smooth shading. Try Shade Flat.

Keep in mind that the Fill command creates NGONS - it's not very good Quad Topology. But I guess it still gets the job done.

Since Blender 3.4.1, in Draw Mode of a GP Stroke, there's a checkbox Outline in the top menu bar under Stroke.

This amazing new feature creates not the center line of the Stroke, but instead creates the perimeter Line (inluding varying thickness from pen tablet):

So, this might be a good start. Depending on your Stroke geometry, you might want to use a Simplify Modifier on it. If you have overlapping strokes, try the Cutter in Draw Mode, then merge by distance in Edit Mode:

Next, you could convert the GP Stroke into a Poly Curve and use a Screw Modifier (with Angle = 0° and Screw = x) on it, so until now, it's still semi-nondestructive:

At last, you would need to convert it to a mesh, which automatically applies the Screw Modifier (make a backup duplicate first!). Then, you can enter the Edit Mode, select the Borders with Alt+Leftclick

and hit F to fill them. When you get weird shading, it's because the original curve had smooth shading. Try Shade Flat.

Keep in mind that the Fill command creates NGONS - it's not very good Quad Topology. But I guess it still gets the job done.