That HAS to be illegal just about everywhere.
So you are looking for a chirped source (A sweep generator)?
It is a hidiously inefficient way to jam anything, especially if you know where the control channels are, but anyway, you can go after GSM much smarter then what you are proposing. .
The cheap thing to do is just an off the shelf VCO (Minicircuits) and a sawtooth generator (Opamp, 555, something like that), but if you must over think a sweep generator then the state of the art in DDS is output from near DC to somewhere around a GHz ( But you will need a 2.5GHz clock to pull that off). DC(almost) to ~400MHz is very mch easier, look to the AD9912 (Which also has a built in squarer, handy if the third, fifth or seventh harmonic can be engineered to be somewhere useful).
The other nice thing about the AD9912 is that it does programable sweeps as well as other waveforms built in, so you can program the thing in advance over an SPI link from your micro and then just send it a command to trigger a sweep.
Not a difficult thing to build, but as I say, illegal as hell and a chirp based jammer is not going to be particularly power efficient.
Regards, Dan.