tomscub - where are you in the country?
At this moment, the ADS-B system does not care whether you have a compliant GPS on board or not when it sees an ADS-B out signal and feeds you back the traffic picture. (That's due to slightly change in 2016, but we'll have an update that will allow SkyView customers that have yet to equip with an SV-GPS-2020 to keep receiving traffic).
But, the short answer is, for now, you should be receiving the same traffic portrait with SV-GPS-250 as you would with a GTN or SV-GPS-2020 as the position source. If that were the case, you should see "ADS-B OK" for the vast majority of the time. So if you're never seeing ADS-B OK and always seeing ADS-B NO RADAR, you may have a configuration problem. A few questions:
What version of SkyView are you running?
In SETUP MENU > TRANSPONDER SETUP, is the top status line yellow? Are all of the other settings correct, particularly your Tail number and Hex code? (you can check your hex code at http://registry.faa.gov/aircraftinquiry/NNum_Inquiry.aspx, then look for the "Mode S Code (base 16 / hex)" box.