glass microfibers are made by silicates (like glass), silica gel is silica SiO2, it is another chemical substance

Both glass and silica gel consist mostly of silicon dioxide. In fact, one can react compounds to create silyl ethers on either glass or silica gel. They both have free silanols.

Silica gel is usually more pure than glass- glass contains other elements such as calcium, sodium, and other materials that affect chromatography, but these are useful to control the properties of glass. Boron is sometimes added to make "borosilicate" glass used for labware and some kitchen glassware. Silica gel is also usually more porous, which provides greater surface area for the interaction with compounds for chromatography.

The silica used in TLC plates is manufactured by dissolving a silica material in caustic followed by titration with acid to precipitate the purified silica. Glass fibers are made by melting the silica material followed by extrusion (I'm perhaps over-simplifying the process).