I guess I don’t know what Realms I want. As a Greyhawk DM (running 3.5e), I would use it as an alternate world, and it sounds like the Gray Box AD&D is the good one (~1357 DR). My PC’s used Dismissal on a semi-friendly NPC and I randomly determined he’s in FR … probably at the Yawning Portal.
Seems like 2e was ~1367 DR, 3e ~1379 DR, 4e ~14xx DR, not sure on 5e, with huge changes for the editions. At least in Dungeon, it seems to matter a lot what DR it is, if it’s after Spellplague or not, certain gods dead or not, etc.
Whereas GH has only the GH Wars changing the setting between AD&D and 2e, with the 3e setting making sense of a 2e messy setting … and frankly the Wars don’t change game play. Most DM’s, including myself, have rejected at least some aspects of the Wars changes. (Not to get too far off topic, for me the concept that wars were all related and conveniently started and stopped together with a treaty with an evil god(!), and that PC actions in the mega adventures TOEE and G123D123Q1 are “fail” and in the past is where I say, um, no.)
For FR, perhaps it’s better to know the 5e version, to keep up with the game? But consensus seems to be AD&D 1e, eh?