GDSI learned of Rontan's tax problems when KPMG performed the analysis which was part of the SPA agreement. This knowledge is the reason General Capital pulled their financing. So, yes, GDSI knew Rontan had major financial problems, and they knew they couldn't afford to close the SPA as they had no means to fund it.
GDSI did nothing for around two years before deciding to pursue this litigation. The tax debt has been discussed extensively in court filings.
GDSI did not know the extent of the tax debt when they signed the SPA.