[rhapsomatrics]

I think,here,that I'd love to agree with "casiopea" to some extent, though it may as well be a "lazy escape route".Tackling this issue from the standpoint of the structural composition of the sentence in reference is sparingly ambivalent.
"to understant"...is best identified as a noun phrase engendered by a preposition.It can also be replaced by "understanding"(semantically)I don't honestly think that "to understand" and "to understanding" are close enough in both meaning and usage to bring confusion.
"To have" a good wife is a great privilege(correct)
"Having" a good wife is a great privilege(correct)
"To having" a good wife is a great privilege(incorrect)
Therefore,it suffices to say that the "to" infinitive is only used with a gerund when projecting into the future with a specific mission.

I am looking forward to(....A......) from you.
I want to(......B.....) from you.

"A" above requires a noun...gerunds are verbal nouns...hearing
"B" above requires a verb....hear
Consentration is essential to.......C....... a comprehension passage.
...Since consentration is a noun,the space marked C should also take a noun...understanding"
I think the best way get this is to separate the infinitive(first) and then consider the structure of the sentence to determine what to use.
.maturity.. is indispensable (to)........ a relationship.
.....the word maturity is a noun,therefore the space should take a noun....
"found"(to establish) is a verb therefore "founding" would be more appropriate in this context....Maturity is indispensable to founding a relationship!