It seems as if you need to get a scene at every approval level, and like you said, you can skip a level or two and incidentally lock yourself out of it. Here's what I found in my run that was romancing Gale:
By the time I reach the Grove I've camped twice, done the Crypt, and get two distinct Gale scenes (Go to Hell & mirror Image scenes) while still being at neutral.
The Grove is where I experienced the worst issues - you can gain a lot of favor too quickly here. Saving Sazza and all it's associated scenes made me skip straight into High Approval, causing me to have to reload a previous save. After reaching Medium approval, I camped a few times and still got nothing! I had to leave the Grove and discover a new area to trigger Gale's third scene. It seems companion scenes are also triggered by regions discovered, but because Gale needs all his scenes at every level to unlock his romance, it's problematic for him.
So basically, I had to do the Grove in increments to be sure I didn't skip levels.
Once reaching the Grove I ask Zevlor about Nettie (but don't meet her yet) and recruit Wyll and camped 3rd time. Companions I think have something to say in regards to finding a healer. Might not be a notable Gale scene here so don't worry.
Did Zorru quest and spoke to Kagha and at that point was at Medium Approval. When I camp here for the 4th time, the scene did not trigger. But when I leave to the bridge unlocking the Forest area and THEN camping, I unlocked Gale's 3rd scene (which I believe is him asking for magical items).
After that, I had gone back to the Grove to do the Sazza scene, learning of Priestess Gut, as well as saving Sazza (but not letting her out the cage), and by that point I got High Approval. Gale approval seems to go nuts for this scene, because you save Sazza and get approval, Gale usually has something to say (the Crossbow in the Tavern conversation) and I believe can gain approval when you talk to Sazza herself when she asks why you saved her.
Again camping immediately after did not unlock another scene, so I left the Grove again and discovered the Blighted Village area which did unlock the Weave scene. I was surprised when I got this, so my specifics may be off, but this was also before Gale died for me and before I had an item to give him.
After this point, having to min-max Gale's approval like this, no one else in the party had much approval, so Gale seemed extremely chatty at this point. I hope they shift Gale's flags to allow for you get to his scenes even if you skyrocket into High Approval as most people are experiencing.
Edit: some things I left out: I was playing a very persuasive good aligned character who tended to succeed a lot of rolls. Never used the tadpole so those scenes never interfered at camp (you can use hte tadpole - it just can throw off camp scene order). Was able to save the kid Kagha tries to imprison. I was using the thread
https://steamcommunity.com/app/1086940/discussions/0/3010053344566355487/?ctp=2#c2969523484447460847 on the steam forums as a lose guide and I think you do have to do two "good guy" feats in the Grove to trigger Gale's scene where he tells you about needing magical items. Doing all this really cemented to me that Gale is neutral good, by the way, his hyperfixation on Mystra aside.
