Bindings contexts for OpenTK 4. If you need to load the bindings for windows use the wgl one, for linux glx and if you use it in a cross platform app like a gtk one use the native bindings context, it will check the platform and use either wgl or glx depending on the platform. For Mac ¯\_(ツ)_/¯ idk, I don't have a mac to test

GlxBindingsContext.cs

using System;
using System.Runtime.ConstrainedExecution;
using System.Runtime.InteropServices;
using OpenTK;

public class GlxBindingsContext : IBindingsContext
{
  [DllImport("libGL", CharSet = CharSet.Ansi)]
  private static extern IntPtr glXGetProcAddress(string procName);
  public IntPtr GetProcAddress(string procName)
  {
    return glXGetProcAddress(procName);
  }
}

NativeBindingsContext.cs

using System;
using System.Runtime.ConstrainedExecution;
using System.Runtime.InteropServices;
using OpenTK;

public class NativeBindingsContext : IBindingsContext
{
  private static IBindingsContext _context;

  public NativeBindingsContext()
  {
    if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
    {
      _context = new WglBindingsContext();
    }
    else if (RuntimeInformation.IsOSPlatform(OSPlatform.Linux))
    {
      _context = new GlxBindingsContext();
    }
    else
    {
      throw new PlatformNotSupportedException();
    }
  }

  public IntPtr GetProcAddress(string procName)
  {
    return _context.GetProcAddress(procName);
  }
}

WglBindingsContext.cs

/*
MIT License

Copyright (c) 2019 Eschryn <https://github.com/Eschryn>

Permission is hereby granted, free of charge, to any person obtaining a copy
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THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/

using System;
using System.Runtime.ConstrainedExecution;
using System.Runtime.InteropServices;
using System.Security.Permissions;
using Microsoft.Win32.SafeHandles;
using OpenTK;

public class WglBindingsContext : IBindingsContext
{
	[DllImport("opengl32.dll", CharSet = CharSet.Ansi)]
	private static extern IntPtr wglGetProcAddress(string procName);

	private readonly ModuleSafeHandle _openGlHandle;

	public WglBindingsContext()
	{
		_openGlHandle = Kernel32.LoadLibrary("opengl32.dll");
	}

	public IntPtr GetProcAddress(string procName)
	{
		IntPtr addr = wglGetProcAddress(procName);
		return addr != IntPtr.Zero ? addr : Kernel32.GetProcAddress(_openGlHandle, procName);
	}

	private static class Kernel32
	{
		[DllImport("kernel32", SetLastError = true, CharSet = CharSet.Ansi)]
		public static extern ModuleSafeHandle LoadLibrary([MarshalAs(UnmanagedType.LPStr)] string lpFileName);

		[DllImport("kernel32", SetLastError = true)]
		[return: MarshalAs(UnmanagedType.Bool)]
		public static extern bool FreeLibrary(IntPtr hModule);

		[DllImport("kernel32", CharSet = CharSet.Ansi, ExactSpelling = true, SetLastError = true)]
		public static extern IntPtr GetProcAddress(ModuleSafeHandle hModule, string procName);
	}

	[SecurityPermission(SecurityAction.InheritanceDemand, UnmanagedCode = true)]
	[SecurityPermission(SecurityAction.Demand, UnmanagedCode = true)]
	// ReSharper disable once ClassNeverInstantiated.Local
	private class ModuleSafeHandle : SafeHandleZeroOrMinusOneIsInvalid
	{
		public ModuleSafeHandle() : base(true)
		{
		}

		[ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail)]
		protected override bool ReleaseHandle()
		{
			return Kernel32.FreeLibrary(handle);
		}
	}
}

As a remark, the wgl bindings context require an active opengl context.
For Gtk I suggest loading the bindings inside the OnRealized event of an opengl area after making the context of it current because wgl requires an active context

Has linux not more then just 1 dll?
https://github.com/opentk/opentk/blob/opentk5.0/src/OpenTK.Platform.Native/DllResolver.cs