While simulation-based glider flight instruction cannot be logged as flight instruction, it can be logged as ground instruction when provided by an authorized instructor.
14 CFR 61.107 (a) – General requires you receive and log ground and flight training from an authorized instructor in the areas of operation specific to the aircraft category and class rating being sought. For our purposes, aircraft category is Glider; the glider category is not subdivided into classes.
Nearly all the areas of operation required for a glider category rating can be learned in simulation. The Flight Lessons available on this site are designed to help you do just that, either on your own or in cooperation with a glider flight instructor or mentor.
14 CFR 61.107 (b) – Areas of Operation is subdivided by aircraft category (and class as appropriate). You need to scroll down to 14 CFR 61.107 (b)(6)(i-xi) – Glider Category Rating for a list of the areas of operation specific to glider flight training.
To supplement your flight lessons, I recommend you get Russ Holtz’s Flight Training Manual for Gliders (FTMG). This publication also contains an excellent flight training progress record, additional copies of which can be downloaded from Russ’ website. Each flight training lesson on this site references one or more sections of the FTMG.