Generic String Encoding Rules (GSER) are a set of ASN.1 encoding rules for producing a verbose, human-readable textual transfer syntax for data structures described in ASN.1.
The purpose of GSER is to represent encoded data to the user or input data from the user, in a very straightforward format. GSER was originally designed for the Lightweight Directory Access Protocol (LDAP) and is rarely used outside of it. The use of GSER in actual protocols is discouraged since not all character string encodings supported by ASN.1 can be reproduced in it.
The GSER encoding rules are specified in RFC 3641 and unlike other common types of encoding rules, are not standardised by ITU-T.
References
Generic String Encoding Rules Wikipedia(Text) CC BY-SA