I use Unity2018.4.28 and TextMeshPro1.5.3, and still find that FontEngine.GetGlyphPairAdjustmentTable function will bring high GC when there are characters added, and the dynamic font method is used, so I cannot find a solution at present.
Adding new characters and glyphs to font assets currently results in 102 bytes of allocations per character and glyph. This is something that I should be able to improve upon in the future.
In terms of the allocations related to Pair Adjustment Records, this is something that will also be improved upon in the next preview release(s) which will be version 1.6.0-preview.x for Unity 2018.4
Automatic fetching of those Glyph Adjustment Pairs can be disabled in the TMP Settings as seen below
