Lobbe brings a wealth of experience to The Rec, the Argentine has been capped 60 times by his country and has amassed 19 appearances in Europe's premier competition, the Heineken Cup.
Van der Gissen led his country in this summer Churchill Cup and the 28-year-old spent last playing with French Top 14 side Albi.
"Both players will be a valuable addition to the squad at this time when we have injuries in the second row," said head coach, Steve Meehan.
"They are both experienced players, who have played in a variety of leagues, and have really good attitudes. I'm sure they will fit right in with the squad we have here.
"Ignacio has played at the highest level for his country, and is a proven leader, so will be bringing some great experience with him, and John is a strong player, who has some very good lineout skills and is also athletic around the field."
The former Heineken Cup winners begin their European campaign against last seasons beaten finalists Biarritz on Sunday, October 10.