Gerunds and gerundives can be confusing because different books use different terminology.... But this is what seems to be most common (e.g. in Wheelock):
1) Gerund: a verbal noun. It only has forms in the oblique cases (for nominative, you use an infinitive instead). So, for a verb like porto, the gerund is portandi, portando, portandum, portando.
"To err is human": errare est humanum (infinitive).
"Love of reading": amor legendi.
"We become happier by living well": Feliciores fimus vivendo bene.
2) Gerundive: a verbal adjective. It has all the inflectional forms: portandus, -a, um. Whenever a gerund would take a direct object, Latin normally uses a gerundive agreeing with the noun instead. You could say, Discimus legendo libros, "We learn by reading books," but it's more usual to say, Discimus libris legendis. (Some might say that this "literally" means "We learn by books being read," but I personally don't find it helpful to remember a "literal" meaning that is almost nonsensical in English, and has a completely different nuance anyway.) So this explains how perficiendis is being used in your Caesar sentence: in perficiendis pontibus, "in completing the bridges." It's equivalent to in perficiendo pontes, but you would rarely say that in Latin.
3) The future passive participle is just another name for the gerundive. One of its most common uses is to express obligation:
hic liber mihi legendus est. "This book must be read by me," i.e. (in real English) "I have to read this book." The subject of the sentence is the thing that must be done, and the gerundive agrees with it. If the verb is intransitive, the gerundive is neuter singular (impersonal). The person by whom the thing must be done is dative.
Caesari maturandum est. "Caesar must hurry."
The sentence in your passage is a form of this, but it's in indirect discourse (with censuit), and esse is omitted (as it frequently is in such situations).
Caesar maturandum sibi censuit, "Caesar decided that he had to hurry."
[Caesari] in pontibus perficiendis periclitandum est. "Caesar must run a risk in completing the bridges."
So the whole thing put together means, "Caesar decided that he had to hurry, if it was necessary to run the risk in completing the bridges, in order to fight a decisive battle before more troops were gathered there." (But this translation doesn't really show that the ut ... dimicaret clause goes closely with maturandum: i.e. he decided that, if he was going to try to complete the bridges, he had to hurry in order to fight a decisive battle before more enemy troops arrived.)
Dic mihi, Damoeta, 'cuium pecus' anne Latinum?