Linking the 3D printing path coordinates to CAD for modelling
The wording in the paper is quite verbose and somewhat unclear. All it says is they read the G-code file and somehow turn it into a 3D model.

A g-code file is just a list of linear moves. Here is an example snippet I took from a random file (keep in mind a typical file would consist of thousands of such lines):

G1 X140.621 Y114.840 E0.0065
G1 X140.804 Y114.765 E0.0129
G1 X141.016 Y114.737 E0.0199
G1 X158.984 Y114.737 E0.6070
G1 X159.196 Y114.765 E0.6140


Each move is relative to the previous, so the second line of the code (for example) tells the printer to move to X=140.804 and Y=114.765 from the previous position (X=140.621, Y=114.840) while extruding an amount of material equal to 0.0129-0.0065=0.0064 mm of filament.

It appears that the authors have developed a toolchain to turn a G-code file into a 3D model, translating every extrusion segment into a part of a solid body (from the pictures, it appears that for a given move segment, they create an ellipsoidal extrusion and merge all of these together into a single solid body) - see Figure 9 in the paper.