Well you have already understood the usage of "koto ga aru". It means having had some experiences of something. This expression is very close to present perfect of English. "koto de aru" is completely different expression. 1. It is said that ~ ex. 走ることは健康に良いことである。 Hashiru Koto ha kenkou ni yoi koto de aru. It is said that running is good for health. I hope this post helps you.

de aru = the couplula and means "desu" or "da" ~koto de aru = it's some kind of fact ga aru = there is koto ga aru...refers to something that there is That is the best I can do without context.