Perceived pitch can be greatly influenced by an improper location of the support rails on which the bar rests, and/or by an unsuitable stiffness of the support material or of the restraint that keeps the bars from falling off. Careful attention here may help quite a bit without having to make changes to the bars.
In general, you would shorten the length of the bar to increase pitch, but the amount of grinding you need to do for any significant change in fundamental pitch can be fairly large. It is important that any significant reduction in bar length be made equally at both ends to avoid moving the "nodal points" off the supports.
If you are using "commercial" Glock bars, they most likely are made from very hard steel or (rarely) brass, and may be plated. Any adjustments to individual bars that you make for tuning will of course require removal of the plating in the affected area. You should also be aware that grinding on highly tempered steel can cause enough heat to affect the temper – and spoil the sound – if you try to remove material too rapidly.
Although he is more interested in Marimba tuning than in Glockenspiel, the principles given at Tuning a Marimba would apply to your tuning problem.
The "pure Glock" tone is essentially that of a rectangular bar that is cut to very nearly the correct pitch. A Marimba bar is meant to have a fairly large "arch" on the underside. The arch is shaped to get different relationships between the fundamental and the first few overtones than is obtained with a pure rectangular bar – the "ideal" Glock bar. The amounts of material that can be removed from the thickness for tuning, without losing the characteristic "Glock" sound is much smaller than is shown for the Marimba bars in this article, but the general areas where material can be removed to adjust pitches is generally the same.
For "fine tuning," the usual method is to start with a bar that is slightly "sharp" and lower the pitch by grinding to reduce the thickness of the center section – usually grinding only on the bottom of the bar. Exactly where you grind affects the relative pitch loudness of the first two or three harmonics, so "practical tuning takes some practice."
If you look about half way down the web page linked, you'll find a picture of a marimba bar with separate areas generically indicated showing which areas affect which of the harmonics, along with the bit of text:
"The fundamental is lowered more than the overtones when material is removed from the center of the arch (area 1 in illustration below). The second transverse mode is lowered most when material is removed somewhere between the center and end of the arch (area 2). The third transverse mode is lowered most when material is removed near the end of the arch (area 3). When all three frequencies are at the desired harmonic intervals, then material is removed evenly across the arch to approach the target note."
The third transverse mode is generally considered insignificant for Glockenspiels, but the fourth transverse mode may be more significant. The general rule is that to lower the pitch you make the bar "less stiff" (thinner) at the point where the "most bending" takes place for the mode of interest.
For very fine adjustments of a bar, the pitch of a Glock bar is sometimes raised by drilling a small hole or two up to half way through a bar to remove "mass" at the place where the bar "moves the most" for the mode you're trying to adjust. Adjustments at this level generally require a very experienced "ear" and/or fairly sophisticated sound measurement equipment. Also, if you have good quality bars, you may find them rather hard to drill without "industrial grade" drill press and bits.
A "professional grade" Glockenspiel is a pretty sophisticated – and surprisingly expensive – instrument. Depending on your use, you may find it more helpful to learn to recite the mantra of free reed and banjo players –