RMK ARNARNES 60 GR 30 G 50KN
Basically means:
RMK = Remark
Arnarnes = The lighthouse as explained before
GR = Degrees. Shortened from gráður which is degrees in icelandic
30G50 = 30 Gusting 50 kts.
This is done basically because the fjord which the airport is in (Skutulsfjörður) is much narrower than the main fjord (Ísafjörður) and the added info can help flighcrew in the decision making as the approach is down the main fjord but then you have to visually fly into the fjord and make the landing which more than often requires a 180° turn in the bottom of it.
Here's a cockpit video from some years ago: