The use of "-gh" in Modern English is largely historical, reflecting older pronunciations that have since evolved or disappeared in most dialects. 
Its appearance in words today is a result of the historical development of English spelling, 
which has preserved these letters as silent markers or indicators of vowel quality rather than as symbols of specific sounds to be pronounced.

Where applicable, Inglish keeps the "h" or replaces "gh" with "ff", but the "g" is dropped entirely in every case.

E.g. 
the sight -> þe sîht /saɪt/
the sigh -> þe syh /saɪ/
the fright -> þe fryht /fraɪt/
the flight -> þe flyht /flaɪt/
the fight -> þe fyht /faɪt/
the knight -> þe nîht /naɪt/
the night -> þe nyht /naɪt/ (influenced by Greek νύξ, latinized "nyx")
sleight -> slyht /slaɪt/
slight -> slîht /slaɪt/
the height -> þe hîht /haɪt/
the weight -> þe ɯeht /weɪt/
the freight -> þe freht /freɪt/

The "h" is kept as a buffer between "y" and "t", or "î" and "t".
The "h" serves both as a buffer between the letters and as an etymological vestige.
Where -eight is pronounced eɪt, the "h" both elongates the "e" as well as serves as an etymological vestige.


Where the -gh is pronounced as "f":

E.g.
cough -> còff, ta còffe
enough -> enoff
laugh -> laff, ta laffe
rough, to roughen -> roff, ta roffen
slough -> sloff, ta sloffe
tough, to toughen -> toff, ta toffen


In some instances, the "gh" is dropped entirely along with other vowels.

E.g.
plough, plow -> plao, ta plaoe
slough -> slao, ta slaoe
thorough -> þouro, or þòro
though, although -> þô, alþô
through -> þru


https://www.merriam-webster.com/dictionary/slough#dictionary-entry-1
https://www.merriam-webster.com/dictionary/slough#dictionary-entry-2

https://www.merriam-webster.com/dictionary/slough#dictionary-entry-3
https://www.merriam-webster.com/dictionary/slough#dictionary-entry-4

https://www.etymonline.com/word/sleight#etymonline_v_23665
https://www.etymonline.com/word/slight#etymonline_v_23674