I often find that when external armor doesn't want to fit together properly it is often the result of either extra plastic somewhere that is interfering, be it a gate nub mark that is just large enough to interfere or something else from the mold design that is not supposed to be there. For the gate marks themselves I would recommend a glass file, either the usual beautycare clear ones or the dedicated hobby ones like Gunprimer Raser. As for your comment about the visor appearing too dark, I would recommend using a chrome silver paint marker (I use Dspaie brand but the latest Gundam EX marker would also work) on the grey internal sensor part, it helps make the translucent visor of GM type mobile suits stand out more. In regards to the parts being too loose, I would suggest trying out any joint strengthening methods such as putting a small amount of glue, blue tack material, etc. on connections and slowly adding or removing more depending on how it fits. I use a "joint strengthening pen" marker from Dspaie that helps me out a lot with the joints on mecha musume kits. Likewise, if a connection is too tight, you might need to lightly sand the connection peg. Again, do it in small amounts and repeatedly test the fit of the parts. I also bought this MG GM Cannon when it appeared on pbandai over here in the USA, albeit it is the "North American Front" color version. It sits with all the other MGs in my backlog simply because I do not have the display space for it.