Here are some lessons I've picked up from running AD&D for my GURPS-centered group. These may serve as reminders for the more veteran AD&D players and lessons ahead of time for the guys new to AD&D. Especially those who've played other, more recent D&D versions such as 5th edition. Going to AD&D from 5th edition is like going from Skyrim to the wireframe labyrinths of Wizardry. It may not necessarily but more difficult, but it's less forgiving and very different.
HP Are Expendable, But Not Easily Replaceable
HP are a resource. You'll expend them to get things done - either by setting off traps you need to get past but can't disarm or fighting foes you can't avoid or deal with without damage.
The issue is, they don't come back easily. Healing spells take up valuable memorization slots, have random healing effects, and aren't very numerous. Healing potions help but it's uncommon to finish an adventure full on HP and with spare to go.
This is in contrast to GURPS, where HP are easily replaceable but very dangerous to trade off for effect.
You can't avoid losing HP in AD&D modules, and trying to get by with zero HP of injury will likely mean you get little if anything done. You must trade them off to get anywhere. But you can't blithely get into a HP-for-HP fight and recover from it to fight another.
Spell Selection and Use is Key
It's tempting, if you're given a choice of spells, to take all of the cool artillery spells and fight-ending magic. But you'll often benefit from some of the less commonly chosen spells. You must use your spells as close to optimally as possible. My players will remember the player who came into White Plume Mountain with only attack spells memorized. He died, and didn't even use them all before that happened . . . and the group was hamstrung by the lack of magic to do anything but directly kill.
And if you're playing a game with pregens with pre-picked spells (cough, cough, A-series), you need to use what's been given to you wisely. Don't spray your magic around; use it as needed and make sure it counts. Spells aren't necessarily given to you to solve a specific situations - this is a free-choice roleplaying game, not that contest in Crewel Lye. I've seen "let's save this spell, it can't possibly be the only way past this situation" and "if we have a spell to do (X) then there must be a situation that demands a spell to do (X)." Rarely that's the case.
Weapon Differences Matter . . . Differently
In GURPS, much of what weapons can do is codified in the rules. Damage, type of damage, reach, ST, bonuses and penalties to hit and to parry, specific use modes, etc. In AD&D, not so much. All you get is length, damage vs. Small/Medium, and damage versus Large. Oh, and weapon speed and armor vs. weapon rules that I don't use (and which are marginal in most cases anyway.)
So in an AD&D fight, you might have a spear vs. your foe's shortsword and be able to equally hit each other in a melee. You're not able to just hold yourself at a longer distance and keep your foe at bay. But equally, nothing says the shortsword and spear can both prod past a portcullis to stab a foe equally well, or that a spear will function well in a tight spot. The DM have a lot of leeway to make sure physical differences matter. Use the natural, reasonable logic of a situation and your gear and it's likely the DM will reward you. If you get too cute ("My trident should be able to pin opponents to the wall routinely, so I do that") you're likely to be disappointed. It wouldn't work in other systems, either.
Hopefully this helps the transition back to AD&D for a bit. I'm enjoying the play and watching my players adjust to the circumstances. They're game to try and I'm hoping my words make their effort more fruitful.