I don't see why you couldn't use a GoPro instead of a regular compact point and shoot camera, like a Canon Powershot ELPH. Travel in Europe can lend itself to wide angle photography very nicely. Although larger, more expansive spaces, like the streets of Paris, may become flatter and more boring with a wide angle lens, vs. the smaller twisty streets of London. The GoPro won't be as versatile for framing choices, and you need to be a wide angle afficianado, but it will be waterproof and far more video capable, with a variety of available mounts and stabilizers.

The sensor is the same size (1/2.3"-format) and the resolution, while a little lower than, say, a Canon ELPH, is still decent at 12MP. The only reason it might not be great is if you plan to make large prints from your images, or you don't like wide angle composition.

You do need to understand that the GoPro choices on field-of-view (how wide a scene you can put in the frame) is done via digital cropping--that is, the image is cropped for the narrower "Medium" view, so you'll be down to only 5 megapixels at that point, which won't be suitable for, say, 8x10 prints, but may still be fine if you only plan on posting things on the web. If you like to zoom in a lot, then a dedicated camera with an actual zoom lens might be a better choice for you.

The iPhoneSE has the advantages of being a phone and networked, so if you wanted to do the social media thing with your images, it's a better choice. And as phones are super-prevalent as camera devices, you'll be less conspicuous shooting with one than with, say, a GoPro mounted on a headband or something. And with the appropriate app, you may actually have more control over the camera than the GoPro gives you, so you can explicitly set ISO and shutter speed.

Obviously, a camera with an full manual (M) mode would let you explicitly set iso, aperture, and shutter speed, and is what serious photographers would prefer. I tend to recommend, for serious photography, that a camera should have M mode, a flash hotshoe, and RAW capability, but vacation snaps aren't that serious. :)

But, the iPhone also has all the same issues to a photographer as the GoPro: a fixed focal-length relatively wide lens, digital cropping, and only 12MP resolution. But it has a much smaller sensor (7.2x crop vs. 6x crop). And that's if you're using the back camera. So, the GoPro is going to be a better choice from sensor size.

OTOH, since you're probably going to need your phone to, y'know, have a phone with you, carrying both devices will give you even more picture-taking options (e.g., you can use the GoPro app to remotely control your GoPro).