The spelling of /g/ is <g> or <gg>, but we also see <gh> in ghoul. The /f/ sound is spelled <f> or <ff>, but we also see <ph> in phrase and <gh> in cough.
This is a contrast between a voiced stop and a voiceless fricative, well separated in the mouth. It does not cause any problems.
The only taboo word pair is bugger/buffer.
Interesting pairs include:
The figure for the mean density is 3.3%. The set makes 123 semantic contrasts, a loading of 50%.
alga alpha algae Alfie big biff bug buff bugging buffing bugged buffed bugs buffs bugger buffer buggers buffers cog cough cogs coughs degrade defrayed diagram diaphragm diagrams diaphragms digger differ diggers differs dog doff dogging doffing dogged soffed dogs doffs egg F egging effing elegant elephant Elgin elfin gab fab gable fable gabled fabled gables fables gad fad gads fads Gael fail Gaels fails gale fail gales fails gag gaff gagged fagged gagging fagging gags fags gain fain gain feign gained feigned gaining feigning gains feigns gait fate gaits fates gate fate gates fates gated fated gate fête gates fêtes gated fêted gating fêting Gallic phallic gall fall galling falling galls falls game fame gamed famed gang fang gangs fangs garish fairish gat fat gaud ford Gaul fall Gauls falls gauze fours gawk fork gawks forks gay fey gays feys gays phase gaze phase gazed phased gazes phases gazing phasing gear fear geared feared gearing fearing gears fears geisha facia geishas facias geld felled ghee fee gherkin firkin gherkins firkins ghoul fool ghouls fools ghoulish foolish gig fig gigs figs Gilbert filbert gild filled gill fill gills fills gillie filly gillies fillies girl furl girls furls girth firth girths firths glade flayed glare flair glared flared glares flairs glaring flaring glaze flays glee flea glees fleas glee flee glees flees glint flint glints flints gloat float gloated floated gloating floating gloats floats gloom flume glooms flumes glory Florrie gloss floss glow floe glows floes glow flow glowed flowed glowing flowing glows flows glower flour glowered floured glowering flouring glowers flours glower flower glowered flowered glowering flowering glowers flowers glue flew glue flue glues flues go foe goes foes goal foal goals foals gob fob gobs fobs gold foaled golly folly gooey phooey gore for gore fore gore four gores fours gored ford gorge forge gorged forged gorges forges gorging forging gorse force gowned found grade frayed grail frail grate freight grated freighted grates freights grating freighting grater freighter graters freighters gray fray graying fraying grays frays grayed frayed graze frays graze phrase grazed phrased grazes phrases grazing phrasing greed freed Greek freak Greeks freaks grill frill grilled frilled grills frills grizzle frizzle grizzled frizzled grizzles frizzles grizzling frizzling grog frog groom Frome ground frowned grow fro grows froze grumpy frumpy grumpier frumpier grumpiest frumpiest grunt front grunted fronted grunting fronting grunts fronts guild filled guile file Guinness finis gun fun gunned fund gunny funny gunwale funnel gunwales funnels gusty fusty gut phut Guy fie hug huff hugging huffing hugged huffed hugs huffs league leaf leaguing leafing lug luff lugging luffing lugged luffed lugs luffs mug muff mugging muffing mugs muffs pug puff pugs puffs rig riff rigs riffs rug rough rigs roughs rugger rougher slug slough slugging sloughing slugs sloughs slugged sloughed snug snuff snuggle snuffle snuggled snuffled snuggles snuffles snuggling snuffling staggered Stafford tog toff tug tough Whig whiff wriggle riffle wriggled riffled wriggles riffles wriggling riffling
John Higgins, Shaftesbury, August 2010.