Gershayim sometimes mark abbreviations ("initialisms" or   "acronyms") in Hebrew, and sometimes signal that an expression is to  be read as a number.  In Anglo-American usage, or to put it another   way, in the English language, an "abbreviation" has to be an abbreviation OF something.  There's a detailed discussion of various   possibilities in HCM, p. 30-33, 45. Zayin-gershayim-lamed is an  abbreviation of "zikhrono li-verakhah."  Lamed-gershayim-gimel is not  an abbreviation in the English-language sense  but is a number (i.e. two numerals), so the rules for romanizing abbreviations do not apply  here.  Subject authorities are constructed by different rules from  those that dictate romanization in the bibliographic record.

Nonetheless, a text (for instance, a title page) to be romanized might actually vocalize the expression--i.e., write lamed-PATAH-gershayim-gimel.  In that case, romanizing "Lag" is probably acceptable, and the ref will have "33."  In such a case, a  500 note "Title page vocalized" or "Title page partially vocalized"   should be added to account for the abnormal romanization.
 
The same strictures hold for tav-resh-yod-gershayim-gimel.  The gershayim mark this as a NUMBER not a word, according to LC   practice-- so the romanization is normally "613," despite the fact that  Even-Shoshan cites the form with two patahs. But if a title page  actually presents two patahs,  romanization as a word is acceptable.