I'm at the end of chap11, I managed to recruit everyone by 2/22, which I think may be the cut date for recruitment pre time-skip.
I searched but did not find good information on how to recruit all students on the GD route, which is surprising because it has its quirks that I'm pretty sure are route specific. I wanted to share some of the things I learned as true in my game, I think they will apply to your game as well but eh, I might be wrong. Anyway, here is what I learned along the way:
General advice
Overleveling your Byleth a little bit is not a bad idea. It's easy to have your strong units be overleveled compared to your army, doing it with Byleth will help increase his stats which is a requirement for recruitment. Just don't overdo it 🙂
Professor level matters, the sooner you get a high prof level, the sooner you have more activity points than you actually need. This is important because you'll need those activity point to grind certain skills in faculty training to recruit the last students. In chapter 9 there is a fishing event where you can fish 2 to 4 fish at the same time, each fish is counted separately for your professor xp, so if you accumulate bait and you are ready to grind all your bait on chapter 9, you can get a really high amount of points. I went from B to A professor rating during chapter 9 because of this trick.
You can get asked by students to join your house if you have a B-Support rating with them, this can even be done voluntarily with save-scumming. Don't stress this much, I had this happen with 2 characters only, you can just ask them every week, at B-rating they relax their requirements quite a bit so it's easy to recruit them.
Buy all the gifts (2 shops with gift, both can be unlocked as early as chap5 by finishing a quest that involves fighting), find all the lost items, every month. Prioritize giving the gifts to the students you pursue that have a lower support level than B.
GD-Specific advices
Ferdinand is a problem. Ferdinand has a time gate on its B-support rating with you. I am not sure when the time gating end, but it ends after 2/22 so you can't rely on him being at B-support level. This is problematic because Ferdinand requires DEX and Heavy Armor. DEX shouldn't be an issue, I was at 23 without trying. At worst check your inventory you maybe have permanent stat boosting items available for dexterity (I had 2). Heavy Armor will likely be an issue, you probably did not train that one on Byleth. Alois and Gilbert are your go-to guys for Heavy Armor training, you can save-scum to get "great" instead of "good" when doing the faculty training, which can help if you are really strapped for time. Start working on Ferdinand early, on my game he refused to be recruited before I got to a D+ rating in Heavy Armor, which takes a while since the base is E rating for Byleth.
Caspar is also a problem. This is exactly the same issue as Ferdinand, he is time gated on his support and you can't reach B-level. Caspar requires STR + Brawl. Unless you got screwed heavily on STR, you should be OK I was at 18 when I managed to recruit him. For Brawl that's another skill you probably did not level =/ your go-to gals for Brawl are Catherine and Rhea, I think I managed to nab Caspar at a D rating in Brawl (might have been D+, don't have my nswitch to verify)
So Ferdinand and Caspar made me sweat a bit but the rest was very straightforward. Hope this information will help some people 🙂