I have transcribed the wills of Margaret Lace, 1828, and John Lace 1832.(on mannin.cloud)John states that eldest son Thomas is to be given responsibility of raising his young siblings, all under 14.
Therefore I suggest that Peg Gelling may not have wanted to share this task, so chose not to marry Thomas Lace. Peg had her own income from her Father's Will. She raised their two illegitimate children, John and Elizabeth Lace assisted by her sister Margaret. John Lace names Henry Gelling as witness to his Will.