This does mean that there is some vulnerability of GPS systems to Space Weather.
The two primary effects of space weather on GPS are:
1. Propagation delay of signals caused by the presence of the ionosphere. Result is increased errors in position and navigation.
2. Loss of signal due to scintillation effects caused by small-scale irregularities in the ionosphere. Result is increased errors due to the decreased number of useable satellites and the possible inability to navigate.
Because of this phenomenon warnings have and will be issued when there is any apparently significant solar flare activity which could affect GPS systems. An example a navigational message concerning solar flare activity will be along the lines:
NAVIGATION AND GMDSS.
1. STRONG RADIATION STORM IN PROGRESS UNTIL ddhhmm UTC MMM. EXPECT INTERMITTENT HF RADIO AND GNSS POSITIONAL INFORMATION. MARINERS SHOULD CHECK THEIR POSITION USING ALL ALTERNATE MEANS AVAILABLE.
2. CANCEL THIS MESSAGE ddhhmm UTC MMM.