* France's 10s/30s yield spread widens to highest since Dec 2014
* Germany, Belgium among potential sellers of long-dated bonds
* Euro zone GDP data could put upward pressure on yields
* Euro zone periphery govt bond yields http://tmsnrt.rs/2ii2Bqr
LONDON, May 16 (Reuters) - The gap between 10- and 30-year French government bond yields reached its widest since December 2014 ahead of a 30-year bond sale seen as the first big test of sentiment since France's presidential election.
France's debt agency has been monitoring the market for a 30-year bond sale via syndication - a system where the borrower appoints banks to sell bonds directly to investors - and started marketing the deal on Tuesday.
Although French government bonds have rallied sharply since it became apparent that centrist Emmanuel Macron would become president ahead of anti-euro, far-right leader Marine Le Pen, investors will be keeping a close eye on this sale.
"The appetite for this deal should be good, but we will be watching it closely because parliamentary elections are coming up and they will determine whether Macron can form an effective government," said ING strategist Benjamin Schroeder.
Macron appointed a conservative prime minister on Monday in a move to broaden his political appeal and weaken his opponents before parliamentary elections in June.
Tuesday's sale will also demonstrate investor appetite for duration, particularly with other long-dated bond sales expected from Germany, euro zone bailout fund EFSF and possibly Belgium.
"The spread between 10- and 30-year has been under steepening pressure since the middle of last month, which we think reflects issuance expectation in the sector," Mizuho strategists said in a note.
Germany is slated to sell 30-year bonds on Wednesday while Belgium has cancelled a bond auction for next week. Analysts said that may be because the Belgian debt agency is planning a 15- or 20-year bond sale this week.
Euro zone bailout fund European Financial Stability Facility is widely expected to sell long-dated bonds this week, and Slovenia is selling bonds maturing in 2027 and 2040 on Tuesday.
The gap between France's 10- and 30-year debt rose to 108.1 basis points on Monday, the highest level since December 2014.
High-rated euro zone bond yields were flat to a basis point higher ahead of the release of euro zone economic output numbers for the first quarter of the year, due at 0900 GMT.
Reuters estimates are for 1.7 percent growth year-on-year, a figure that could put further upward pressure on yields.
The yield on Germany's 10-year government bond, the benchmark for the region, up 1 basis point to 0.43 percent.
The lower-rated Southern European bonds outperformed, with the yield on Spanish, Italian and Portuguese 10-year government bonds down 1-3 bps.
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(Reporting by Abhinav Ramnarayan; Editing by Hugh Lawson)