Hello mate and welcome.
There are various ways to play a device through a boombox.
If it has a dedicated RCA line in it is simple to get a twin rca to 3.5mm lead and hook up almost and device with a 3.5mm jack. If there are no true RCA line in there may be a 5 pin DIN line in and leads exist for these to but bear in mind these sometimes require the play key to be depressed on tape deck. If there is what looks like an RCA line in but has "Phono" on it you may need an attenuator to adjust the frequency to make it sound proper.
You can also get an FM transmitter, these plug in to a 3.5mm jack (ipod, laptop etc) and will transmit around your house and you can tune your box to a unused fm frequency and listen. I have one made by a company called Crane and it is very good.
THe GF range you mention often have "Phono" style line in (GF9090 and GF9191) so need an attenuator but others in the range have a proper RCA line in. Let us know any boxes you are interested in and we can advise. There are plenty of ways of playing to them though