Great. But I prefer if, else in golang instead of shell. Could you change like this

if os.Getenv("XDG_SESSION_TYPE") == "x11"
...
else
...

Also, please add wl-copy in Dependencies of README

For sure! Just realised perhaps I should have opened this as an "enhancement" issue before opening the PR, since I don't know anything about the Go language, my bad.
I'll give it a try though, if I manage I'll update the README too.
If I can't, apologies in advance, and you can close this, I can open it as a suggestion/enhancement in an issue, would that be OK/better?

UPDATE

Ok, so I've reading up and trying things, and I think I got it to work with native go syntax, could you give it a look?
It's working for me on Sway (Wayland)

package main

import (
	"fmt"
	"os"
	"os/exec"
	"runtime"
)

func IndexOf(candidate string, arr []string) int {
	for index, element := range arr {
		if element == candidate {
			return index
		}
	}
	return -1
}

func SetTermTitle(title string) {
	print("\033]0;", title, "\007")
}

func CopyToClipboard(text string) {
	switch runtime.GOOS {
	case "linux":
        if os.Getenv("XDG_SESSION_TYPE") == "x11" {
		exec.Command("sh", "-c",
			fmt.Sprintf("echo -n '%s' | xclip -selection clipboard", text)).
			Start()
        } else if os.Getenv("XDG_SESSION_TYPE") == "wayland" {
                exec.Command("sh", "-c",
			fmt.Sprintf("echo -n '%s' | wl-copy", text)).
			Start()
	}
	case "darwin":
		exec.Command("sh", "-c",
			fmt.Sprintf("echo -n '%s' | pbcopy", text)).
			Start()
	case "windows":
		exec.Command("cmd", "/c",
			fmt.Sprintf("echo %s | clip", text)).
			Start()
	}
}

I added "os" to import and instead of my original pull I defined Wayland as an option too, just in case some other display server exists/might be introduced (although I'm not really aware of any other (major) ones...). I don't know if I could have done it a little cleaner.
Finally, if adding wl-copy to the README I would add it as wl-clipboard, and as "wl-clipboard for Wayland"; should I edit the xclip entry too?
Maybe
"Linux
xclip for X11
wl-clipboard for Wayland"

or

"(Linux) xclip for X11
(Linux) wl-clipboard for Wayland"

What do you think?

Nice, your patch works for me.
But now I think maybe it can use switch-case instead of if-else. It prevent wayland user call os.Getenv("XDG_SESSION_TYPE") this function twice.

Finally, if adding wl-copy to the README I would add it as wl-clipboard, and as "wl-clipboard for Wayland"; should I edit the xclip entry too?
Maybe
"Linux
xclip for X11
wl-clipboard for Wayland"
or
"(Linux) xclip for X11
(Linux) wl-clipboard for Wayland"

I prefer the latter.

Great! So you wanna change switch-case for if-else?
Sorry, wouldn't have any clue how to do that. As I said, I have 0 Go language experience 🙁️.
How would you change it to switch-case?
In the mean time I've already modified the README, I'll make a PR once this is fixed.
In the end I kept it more in-tune with your original format and style, so as to not seem out of place:

https://github.com/muvment/GTT/blob/2771c1baa3af0b97ed3affcb427c06c073e1ed43/README.md?plain=1#L72-L78

The switch-case syntax should like

switch os.Getenv("XDG_SESSION_TYPE") {
case "x11":
...
case "wayland":
...
}

In the mean time I've already modified the README, I'll make a PR once this is fixed.
In the end I kept it more in-tune with your original format and style, so as to not seem out of place:

https://github.com/muvment/GTT/blob/2771c1baa3af0b97ed3affcb427c06c073e1ed43/README.md?plain=1#L72-L78

It looks so great.

Ok, let me have a look.

Something like this?

	case "linux":
	    switch os.Getenv("XDG_SESSION_TYPE") {
        case "x11":
        exec.Command("sh", "-c",
			fmt.Sprintf("echo -n '%s' | xclip -selection clipboard", text)).
			Start()
        case "wayland":
        exec.Command("sh", "-c",
			fmt.Sprintf("echo -n '%s' | wl-copy", text)).
			Start()
        }

edit

It's working for me on Wayland

Yes

Great! I'll change it now

Lol, who would have thought it would be that simple, it took me just a minute, I was thinking it would take me another hour or so 😂️.
Anyway, done. If it's good I'll make the PR for the README.

Btw, how you comment snippets of code with syntax colour highlighting?
And sorry for the title change mistakes, I'm new to this, the UI options confuse me a little still.

Btw, how you comment snippets of code with syntax colour highlighting?

Add the language to the end of first line backquote, eg. ```go

Cool! Thanks! And thanks for the Go info, I ended up learning some Go along the way. In any case, much appreciated and have a good one 💪️🙏️