Modrinth in the same launcher
The CurseForge App is, by design, CurseForge-only. Modrinth has been growing fast, especially for Fabric mods, performance mods, and shaders, and many authors now publish to both platforms. GDLauncher's built-in browser searches both at once, so you don't have to pick.
Server management
GDLauncher includes built-in Minecraft server management, create a Vanilla, Forge, Fabric, NeoForge, or Quilt server and manage it from the same UI as your singleplayer instances. The CurseForge App doesn't include server management.
Cloud Instance Sharing
Both launchers can share a setup with a friend. The CurseForge App keeps everything inside the CurseForge ecosystem, you can hand off a CurseForge modpack but a setup that mixes CurseForge mods with Modrinth mods can't travel intact. GDLauncher's Cloud Instance Sharing accepts the mixed case: paste one code, the recipient gets your exact instance with files from both platforms re-downloaded from their original CDNs.
The verdict
If you only ever install CurseForge content, the CurseForge App is the official choice. GDLauncher gives you the same CurseForge integration plus Modrinth in the same browser, Cloud Instance Sharing that travels with mixed CurseForge + Modrinth setups, and built-in server management.