Etymology

Formed on gayn (adj.), cp. further OIcel gegnliga 'suitably'.

PGmc Ancestor

*gagni- 

Proposed ON Etymon (OIcel representative)

gegn ‘straight, direct, ready, serviceable, kindly’, cp. gegnliga 'suitably'
(ONP gegn (2) (adj.), cp. gegnliga (adv.))

Other Scandinavian Reflexes

Icel gegn, Norw gjegn, ODan gjen, Dan dial gjen, Sw dial gen

OE Cognate

gēn (adj.) ‘direct, short’ 

Phonological and morphological markers

absence of palatalization of */ɡ/

Summary category

A1*c

Attestation

Attested from early ME in various dial.

Occurrences in the Gersum Corpus

Gaw 476, 1297

PS suggest the sense 'wittily' for the instance at Gaw 476; McGee 331 understands the instance at Gaw 1297 as an adj., meaning ‘courteous’. 

Bibliography

MED geinlī (adv.) , OED gainly (adv.) , HTOED , Dance gayn (c)(adv.), Bj. 151.