Designing High Rate ADC Using Lowers
Since your question is a bit unclear on the specifics (what do you mean by 1 Gbps signal?  What do you mean by 250 Mbps ADCs?), I am going to make a few clarifying assumptions: DC - 1 GHz signal, and 250 Msa/sec ADC.

If you want to sample a 1 GHz bandwidth signal (DC to 1 GHz) with 250 Msps ADCs, then there are cerainly ways to do that.  To satisfy Nyquist, you need at least 2 Gsps, or eight 250 Msps ADCs.  

If your ADCs have an analog bandwidth of at least 1 GHz, then all you need to do is split up the input signal into 8 copies, feed each ADC with its own copy, and drive the ADC clocks from the same PLL with 45 degree offsets.  This is called time interleaving, and it is used all the time to sample high bandwidth signals.

If your ADCs don't have a 1 GHz input bandwidth, then you can put samplers in front of the ADCs that have at least 1 GHz bandwidth and drive the samplers with 45 degree offset clocks.  This ends up accomplishing exactly the same thing, but allows the use of ADCs with lower bandwidth.  

If you can't find (or make) samplers with 1 GHz bandwidth, then there are other tricks that you can play.  One of them is bandwidth interleaving.  This requires slicing and dicing the input signal in frequency using bandpass filters or diplexers, then downconverting each band with mixers and converting them with ADCs.  This technique requires a significant amount of post-processing in order to undo the frequency conversion and diplexing to get back the original, full-bandwidth signal.  This technique is used in applications that require sampling extremely wide bandwidths, such as LeCroy's 100 GHz bandwidth oscillscope.  That scope uses a triplexer to split the input signal into 3 bands, which are then sampled by the scope's normal 36 GHz bandwidth / 80 Gsps converters.  Internally, each of these 36 GHz bandwidth channels use time-interleaving of multiple ADCs to achieve the 80 Gsps sample rate.  Then software is used to reconstruct the original input signal, at an equivalent of 240 Gsps.  

Now, if by 1 Gbps signal, you mean some 1 Gbps NRZ serial data, then you probably only need around 1 GHz of bandwidth and the rest of my answer still applies, scaling numbers accordingly.  If you mean some sort of higher-order modulation (PAM, PSK, QAM, etc.), then you'll probably need less bandwidth, but you would need to provide some more details in order to get a relevant answer. If you mean some sort of RF channel that contains 1 Gbps of data, then this will probably require downconversion first, and then sampling based on the bandwidth of the signal, which is dependent on the modulation format, channel spacing, etc.