Can I show multiple GPS-data from my holiday on a combined, single map?

If they were contiguous then you can use fitfiletools on the interweb to join them. 

Komoot premium offers this - you can group activities in collections. It’s a paid service, though../

I currently don't have my computer with me, but as far as I remember Garmin Basecamp can do that.

Garmin Explore can also do that (web and app).

My favorite app for route planning is Locus Maps, it can also display multiple tracks.

Some of those GPX online viewers are also capable of that....

If you use Strava it has got a personal heatmap

I think loading up Garmin Explore will show all your tracks while on holiday on the same time.... 

Thank you all for your valuable tips. I have been playing around with the mentioned options. Basecamp actually works really well for this (I hadn't used it in years tbh). Much appreciated, I love the community here!

On the computer simply put all the tracks into one Garmin Basecamp folder, then its easy to display everything in that folder on the map. 

You can also export this folder with all the separate .gpx tracks a single gpx file for display in any other app. 

(I find BaseCamp a great tool for storing planed and completed tracks for reference. Though GPX Editor/Viewer on the Mac is quite good too)

On my iPhone and iPad I use the GPX Viewer app to look at both routes to do and done. Handy to share while out and about or to refer to as needed. I save the combined gpx track, or separate ones if I prefer, to a folder on my iCloud from the desktop computer, then the file can be opened from iCloud by the phone or iPad. Other apps can display varied map backgrounds such as satellite view, OSM, etc.