To add, while it may not be the problem experienced by the poster, I had a similar issue when connecting to a remote server. When connecting to a remote server, the syntax looks like:
>gsec -user SYSDBA -password masterke -database <Server>/<Port>:<FDB>
where I erroneously specified the <FDB> as the name of my database, similar to what is done when using ISQL. This results in the following error:
GSEC> display
invalid request BLR at offset 37
table USERS is not defined
However by simply specifying the Security2.fdb then there is no problem:
>gsec -user SYSDBA -password masterke -database <Server>/<Port>:Security2.fdb
Note that when running gsec on the server itself, only the credentials need to be used - no '-database' option is required.