You do not need to use the web just to look at a gerber file.
KiCad has a bundled in Gerberviewer, called “Gerbview” 
and this is a decent Gerber viewer, and can also load drill data.
It is possible to modify drill data with a text editor, but it’s error prone, and “volatile”, as your edits will be overwritten if you generate the Gerber / drill files again. If you click on your hole, and then press e for edit you probably get a popup window, and can possibly change the drill diameter.
Also, when you place a footprint (of for example a test point) on your hole, there is a good chance that it recognizes your hole as a snap point, and coordinates will be the same. I made extensive use of this in:
KiCad-nightly V5.99 also already has a: Schematic Editor / Place / Add Image and this can load a background image in Eeschema, which can be useful for retracing a schematic.
Here a simple screenshot of an experiment. I imported a B/W picture of some random schematic, and placed a few KiCad schematic symbols over it:
It works, and it saves some time with looking at different representations of the schematic during a duplication effort.
Something similar for Pcbnew would indeed be very nice. Import some color images from a PCB (converting to BW looses too much info). Images from both front and (mirrored) back, Some even go as far as sanding to get to bare copper or even inner layers, so a “bunch” of pictures, and with transparency would be real nice.
If you like this, then go up vote: