Faith is trust that goes beyond what we can prove: trust in a path, a teacher, a principle or the Divine. It is not the same as blind belief, and the Gita does not ask for that. It treats faith as a living quality that grows when it is practised and checked by experience, and that shapes who we become.
Gita 4.39 says faith, sincere effort and mastery over the senses lead to knowledge and lasting peace, so combine belief with disciplined practice. Gita 4.40 warns that persistent doubt joined with ignorance and lack of faith obstructs peace, and Gita 9.3 says faith is the doorway to progress, so cultivate trust in a chosen path and commit to it steadily instead of drifting between doubts. Gita 17.3 says faith follows the character of one's mind and shapes a person in turn.
The Gita also addresses faith in others and in understanding. Gita 7.21 says God honors and strengthens every sincere faith, whatever form it takes, so respect others' sincere paths. Gita 9.23 says sincere faith directed toward any form ultimately reaches the one Divine, yet worship lacking understanding does not give full realization, so let your faith be supported by understanding and not habit alone. Gita 7.22 acknowledges a higher source behind the results of faithful effort, and Gita 17.28 says that effort without faith comes to little.