From what I see in the link you provided, GMI are primarily icons, i.e. images. You can then paste them as images in your document and apply the usual position adjustments (anchor mode, wrap mode, position relative to anchor or some reference, …).
There is also a font. This is the .ttf file you mentioned. You don’t install font in LO, you install fonts into your system and LO uses whatever the OS hands it over. How to install a font depend on the OS. You didn’t tell which is yours; I can’t help more than recommending you follow your OS method for font management.
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