Check Steam Input - for Player 2 to work, Steam Input must keep controller inputs as controller inputs. By default, it converts controller input to keyboard/mouse input. Applying the template by Valve intended for games with built-in controller support should work. Click the controller icon on the Garry's Mod library page to get started.

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Check the USB cable (if applicable) - ...using a wireless controller plugged in with a USB micro-b/USB-C to USB-A cable? Unfortunately, some of these cables are for charging phones only; some of these cables don't support data.

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Ensure that the controller is connected before starting Garry's Mod - Garry's Mod doesn't detect newly connected controllers while running.

引用自 Zroytz：

Check Steam Input - for Player 2 to work, Steam Input must keep controller inputs as controller inputs. By default, it converts controller input to keyboard/mouse input. Applying the template by Valve intended for games with built-in controller support should work. Click the controller icon on the Garry's Mod library page to get started.

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Check the USB cable (if applicable) - ...using a wireless controller plugged in with a USB micro-b/USB-C to USB-A cable? Unfortunately, some of these cables are for charging phones only; some of these cables don't support data.

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Ensure that the controller is connected before starting Garry's Mod - Garry's Mod doesn't detect newly connected controllers while running.

I made sure that it was the default, the usb cable was right, and that it was connected beforehand and it still doesnt work. what am i doing wrong?

Are both the controller & keyboard/mouse controlling Player 1 while nothing controls Player 2?
The default configuration for Steam input in Garry's Mod converts controller input to keyboard/mouse input; it must be reconfigured via the controller button on the Steam library page for Garry's Mod.

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...using a non-Xbox controller & getting no response to controller inputs?
Check Steam settings -> controller; confirm that the switch for the style of controller in use is on.
Also verify that Steam input is enabled for Garry's Mod by clicking the gear on the library page for Garry's Mod -> input.

im using a controller using bluetooth is it good?

I've only tried a wired XBox™ 360 controller (Windows™/Linux) & XBox™ One controller (Windows™ only), but any controller compatible with Steam® Input & your operating system should work. Make sure Garry's mod responds to the controller in general, like just in Sandbox. If, in Splitscreen for Sandbox, the controller is just controlling Player 1 while Player 2 does nothing, make sure Steam® input is configured to keep controller inputs as controller inputs. See the image carousel on the Splitscreen for Sandbox Steam® workshop description page for how to do that.

HOW CAN I GET TO THE INPUT