Can I translate an App from Android using GPLv3 into Swift? Because actually I don't use any source code. Do I need to keep it under GPLv3 and open source? Is there any method to avoid infection if it must under GPLv3?
The GPLv3 licence of the original requires that, if you create a work which is (in copyright terms) a derivative of the original, and you distribute your derivative, you must do so under GPLv3.
It's not clear to me in what sense you're using translate. If you mean it in the linguistic sense, translating from one human language to another, then I can't see any way not to create a derivative.
If you're using it to mean converting an application from one programming language to another, this is normally referred to as porting (or sometimes, as Brandin points out below, cloning). It is possible to do this in a way that doesn't create a derivative work, but it requires two people, and is known as a clean-room reimplementation. To do this, one person must take the current work apart, and express exactly what it does in a document called a functional specification. The other person then, without ever seeing the original work or directly communicating with the first person, implements the functional specification in the new computer language. This is an enormous amount of work. If you decide to do the port by yourself it is very likely that the work you create would be regarded as a derivative of the original work.
Edit in response to Amon's comment below: I do not think there is any justification for the "of course" tossed in at the top of that comment. There is one reliable way not to get sued for copyright violation, and that is not to infringe someone's copyright; everything else is just argument about how effectively one has done this. The advantage of the clean-room reimplementation is that, when done properly, it has already been held to exclude the possibility of violation (with, eg, Phoenix Technologies' IBM BIOS clone, and VTech Apple II ROM clones). In NEC Corp. v Intel Corp, the court held that the similarities between source and final product were a necessary consequence of the product's nature, and thus not a violation, because clean-room techniques had been used.
Any lesser porting technique leaves the defendant in the difficult position of trying to prove that they have not violated, instead of having a nice set of persuasive case law that moves the burden of proof of violation back towards the plaintiff.
MadHatter names one viable way. Another method is to create an emulator. If your Swift emulator emulates enough of the Android runtime, it can run the orignal App (and likely at least a few more Apps, especially the simpler ones).
Because the emulator is a stand-alone work, it's not in any way covered by the original license. Downside: this is hard.