https://www.reddit.com/r/GPURepair/comments/smlu0q/list_of_gpu_repair_resources_schematics/
Vlab.su: Russian forum for electronics repair, has GPU section with schematics and boardviews + tools like nvidia mats but you need to login and contribute to be able to download them.
Badcaps.net: English forum, also has some schematics and boradviews and also requires signing up.
Schematic-X: Free publicly available schematics and boardviews for some graphics cards.
TechPowerUp: The largest VBios library.
TUTORIALS:
Repair.wiki (Nvidia/AMD): Diagnostic tutorials for Nvidia and AMD cards written by me.
A.S.Reparis (YT): My own GPU and other computer parts repair channel.
MV TechLabs (YT): Youtube channel for GPU Repair.
MUST HAVE TOOLS:
Multimeter
Hot Air Station
Soldering station
NICE TO HAVE TOOLS:
DC Lab Bench Powersupply
Dedicated test bench with riser
Stencils for GDDR5/5x/6/6x memory chips
BGA Rework Station for GPU replacements
Microscope
This is by no means a full list, feel free to contribute resources in comments.