I've recently DM'd Curse of Strahd, and have read House on Gryphon Hill, in fact I read through it mid-campaign to see if any of it could be used to augment Curse of Strahd, or in fact mined for any kinds of ideas etc. What I thought was that the adventure is so different, and not really compelling from a DM or player perspective.
I honestly can't imagine running it straight after Curse of Strahd would be much fun for players, or as a DM. Curse of Strahd is a good length, we did it in around 9-10 months only playing two 3-4 hour sessions per month. It has a nice ending, no-one (especially the players) felt any need to play those characters again, basically all the PC's retired happy.
If you really wanted to run House on Gryphon Hill, I'd say it's going to be best run quite some time after Curse of Strahd, with different PC's. I say "quite some time after Curse of Strahd", because the style of the adventure is quite different, so I imagine it's best to return to some familiar aspects (Strahd), in a totally different context, once the memory of CoS has faded somewhat from people's memory.
p.s. the original module is basically modeled on the first few chapters of "Dracula" i.e. when he's still in his homeland. House on Gryphon Hill is the rest of that book, i.e. when Dracula is in London. Curse of Strahd is a very large expansion of the original module - similar in theme, but greatly expanded in terms of the setting, the NPCs, back-story etc; hence why, in my mind, House on Gryphon Hill really doesn't fit any more as a sequel...