Hewitt (2011:13-14)

 "In Standard Breton and in L, the infinitive is dleoud, stem dle-, derived from dle ‘debt’, 
 cf. Welsh dyled ‘debt’ and modal auxiliary dylai ‘should, ought’, and Irish dlí ‘law’. 
 
 The initial consonant cluster dl- is unstable in Breton; three resolutions are possible: 
 (1) insertion of an epenthetic vowel, as in G: dële ‘debt’, dëleein ‘owe; must, ought to’; 
 (2) loss of d-, as in dluzh ‘trout’ > T luzh; and 
 (3) dl > gl, as in widely in T and K: gle ‘debt’, gleoud ‘owe’. 
 
 On the analogy of dle > gle, the ‘mixed’ (lenition/provection) mutation applied to the stem dle-:
 …e≠tlean [AFF I.must/ought], the tl- cluster became kl-, and then this was interpreted as being 
 the basic, unmutated form for the modal auxiliary but not for the lexical verb, so in T there is 
 now gleoud, gle-, ‘owe’
 
 (43a) gleoud a ran arc’hant dit 
       owe.INF AFF I.do money to.you 
       ‘I owe you money’
 (43b) me a =c’hle arc’hant dit 
       I AFF oweº money to.you 
       ‘I owe you money’
 
 but klea’ , kle- ‘must, ought’
 (44a) klea’ a ran mond 
       must.INF AFF I.do go.INF 
       ‘I must go’
 (44b) me a =gle mond 
       I AFF must go.INF 
      ‘I must go’
 
 Guillaume Floc’h informs me that in Plouhineg, south of Gẘaien (Audierne), the lexical verb is 
 lea, le- < dleoud, dle-, but the stem for ‘must, ought’ is tre- < tle-. 
 So in many areas of Brittany, a morphological distinction is made between the stem for the lexical
 verb ‘owe’ and that for the modal auxiliary ‘must, ought’."