It probably won't come as a surprise to you, but it doesn't seem like there are any pre-existing solutions which can link your devices together. However, since you said you were interested in custom-built solutions, you might be able to get close to what you want.
I did a bit of research into your devices to see if they had open APIs you could take advantage of. Unfortunately, not all the devices you listed are open - the Samsung Gear uses S Health, and I could only find APIs to import data into S Health, not get it back out into your own application. The Wahoo doesn't seem to have a simple web API, and nor does the Muse and Dexcom monitor. The rest of the devices do have nice APIs (which I've listed below).
Creating a Dashboard
This really depends on your technical ability with programming. If you're not at all familiar with programming, your best bet would be some sort of spreadsheet software such as Excel or Google Sheets.
Google Sheets supports basic scripting, and someone has developed a way to make JSON requests (the type of request needed to communicate with the APIs I listed). You could also read Is it possible to send HTTP request from inside Google docs? on Stack Overflow for more context here.
Once you get the data imported into your spreadsheet, it should be relatively straightforward to make a dashboard out of the data, and won't require much special knowledge. You'll also get complete design control for what you want to show, which is great, although it will obviously still be a spreadsheet.
If you need any more specific guidance, you can always ask a more specific question (programming questions can also be asked at Stack Overflow). Good luck!
In responses to your questions that you asked in an answer:
Assuming that I know a bit about programming, how realistic reverse engineering would be in making an API for the Gear, Muse and Dexcom monitor? What would be the simplest approach to do so?
Probably not particularly easy; I would contact the manufacturers (as suggested by Sean Houlihane in the comments).
Any idea on how can I get similar results as above? Do I have to design the interface in Photoshop first and then create an HTML page to import my data from Google Sheet or AM I getting it wrong?
It depends on your approach. If you're able to design it in HTML, go ahead and do that, then use a framework like jQuery, Angular or ReactJS to connect your UI to the APIs.
I might have misunderstood but it does not seem like Google Sheets nor Excel enable auto synch for the devices. It sounds like I have to enter the data manually...
That's true, you would have to manually rerun the request in Google Sheets. If you used a HTML page like I mentioned in response to the last question, that wouldn't be a problem as data would have to be fetched new each time from the API.
You could either go the HTML route or the Google Sheets route; if you're able, the HTML method would work a lot better and give you more flexibility.