The CurseForge gap
The biggest difference is straightforward: the Modrinth App can't install CurseForge content. For mods that are Modrinth-only, this doesn't matter. But CurseForge still hosts the larger modpack library and many older Forge mods exclusively. GDLauncher's browser shows both platforms in one search, so you can pick whichever has the version you need.
Both ecosystems are great
Modrinth has a smaller library but a faster, ad-free site and better APIs for modders. CurseForge has the deeper catalog and historical packs. Most popular mods are now on both. GDLauncher's strategy is to support both natively rather than force you to choose.
Server management
Modrinth's server management is the paid Modrinth Hosting integration: you provision a server through Modrinth and manage it from the app. GDLauncher's server management is local, create a Vanilla / Forge / Fabric / NeoForge / Quilt server on your own machine, watch the live console, and edit the same instance settings you use for singleplayer, no hosting bill required.
Cloud Instance Sharing
The other GDLauncher feature the Modrinth App doesn't replicate. Paste a code, get the exact setup with mixed CurseForge + Modrinth content in a single share.
The verdict
The Modrinth App is fantastic if you live entirely in the Modrinth ecosystem. But many of the most popular modpacks (RLCraft, ATM10, DawnCraft, the FTB lineup) are still CurseForge-only, and even cross-platform packs are usually CurseForge-first. GDLauncher gives you Modrinth plus CurseForge in one browser, plus Cloud Instance Sharing for friends, plus built-in server management. Pick GDLauncher if you want the broader ecosystem; pick Modrinth App if you want a focused, Modrinth-only experience.