The Grenade Launcher is hardcoded to 2.5 by default ("temp1 0"). Adding a negative value subtracts from this number, causing instant explosions at 2.5. A positive value is added to this 2.5 value and will thus take that much longer to explode.
There is a variant of this modification known as GTIMERF. This is a closed-source version of GTIMER with a PROGS.DAT file. The open-source version does not come with a PROGS.DAT, meaning it must be compiled out by the user.