Hello Taudelt,
I have some good news and I have some bad news. The good news is that the outline appears to resemble a 'GL', but it is slightly 'blurry'; the bad news is that magazines of 'GL' pistols and carbines were not hallmarked in my opinion. In addition to the two 'GL' Lugers which I own in my personal collection (you may visit my web site hereunder to preview those two pistols), I have examined a few other genuine pistols and presentation carbines which have the same pattern - no 'GL' hallmark on the magazine. My 1900 Swiss Acceptance Luger which has an unrelieved grip/frame and a flat button magazine (this was the the original magazine which was issued with the pistol), has no 'GL'. Furthermore, the magazine which is inserted in the 1898 Borchardt-Luger serial #5, is actually serial serial #10, has no 'GL'. Therefore, using a rule of consistency, I doubt magazines of 'GL' pistols were hallmarked. It is very possible that somebody will add a 'GL' to the wood base of an early magazine to inflate the price, or simply boost that they own a 'GL' pistol with a 'GL' magazine - to me it makes no difference as long as the magazine is correct in a 'GL' Luger.
Otherwise, you have a fine condition early magzine.
Albert