The impact of the US designation of the IRGC as a terrorist organization depends on the Trump administration's ultimate implementation mechanism. Will the US hinder IRGC's financial dealings or confront it militarily as a terrorist entity akin to ISIS or Al-Qaeda? If the Trump administration is considering the former, the impact on IRGC's activities will be relatively minor. There have been various US Treasury sanctions imposed against the IRGC since 2007, but Iran's paramilitary force has managed to operate effectively regardless.
However, it is a different story if the aim is to treat the IRGC and its external special forces--the Qods force--as a terrorist organization. US troops in Iraq, Afghanistan, or other parts of the region may be forced to confront the Qods force in combat, a move that will not go unanswered by Iran. The IRGC Commander has already stated that if the news of the potential terrorist designation is correct, the Guard will reciprocate by considering "the American military all over the world, especially the Middle East, as equal to ISIS." This scenario presents an extreme amount of risk for both the IRGC and the US troops in the region.
Politically speaking, the IRGC will be the ultimate winner in Iran. Early signs indicate unanimous support for Iran's paramilitary force. President Rouhani who has often publicly criticized the IRGC for its involvement in the country's politics and the business sector announced his administration's support of the Guards. Also, the IRGC is currently enjoying a boost of popularity and public support. External threats such as ISIS, have resulted in public's acknowledgement of the entity's invaluable efforts to ensure Iran's security and territorial integrity. Any potential sanctions against the IRGC will unify the country's various political factions in opposing external pressures.