Gimli's prior sight of the Mountains of Moria
 

As the Fellowship entered Hollin, Gimli stated that he had seen the Mountains of Moria from afar "once before in waking life".
Presumably that would have been on the east side of the Misty Mountains, in Wilderland. But what would Gimli have been doing so far south, probably at or even below the Gladden Fields and not too far from Lorien and Dol Guldur ?

Gimli was 139 years old when he said that. Some Dwarves apparently travelled a good deal: Thorin met Gandalf on the road near Bree, and Dwarves regularly passed through Bree: there were some there the night Frodo and his friends met Strider. In 139 years, Gimli might well have travelled either through the Vales of Anduin or even up the old Númenórean road through Enedwaith Eriador, across the ruined bridge at Tharbad and up the Greenway. Either way, he’d have seen the Mountains of Moria at a distance.

We should also consider that Gimli’s remarks were in response to Pippin’s seeing them for the first time when the Company of the Ring first crossed the Hollin Ridge into ancient Eregion: they were 70 or 80 miles away, which must have been close to the limit of what they could see along the horizon, even looking at mountains from the top of the ridge. That probably means that Gimli had seen the mountains from the east side, somewhere in the Anduin basin and possibly closer to them than the Hollin Ridge, since the Old South Road/Greenway was even further away from them than the ridge.

So you must be correct. Assuming that Gimli was not on the west side of the river – the Lórien side – and that he was trying to avoid Dol Guldur as best as possible, the most likely place from which he’d seen the mountains was near the Anduin, west of Dol Guldur, just north of the eaves of Lórien, and for some miles north of that point along the river. And since the Dwarves mostly used the Dimrill Dale entrance to Khazad-dûm historically, that was probably what was usually depicted in the artwork he mentioned.

The Dwarves, at one point (although it was a long time ago), have settled in Dunland. It was around 2790s, after the fall of Erebor, around the time of the war with the Orcs. It was still a long time before Gimli was born, though. But it is possible that some of them might have returned there, and especially given that he seemed to be well-traveled, why not. And it would have been possible for him to see the mountains of Moria from somewhere around there, I think? But that is just one theory that has occured to me.

Blue ridge mountain
 

Oh, and there's one journey that Gimli must have taken at one time or another, from the Blue Mountains, where I imagine he was born, to Erebor after the events of The Hobbit.

But I'd agree that he was probably quite widely travelled as far as the North of ME goes.