Whether Gao Rongchang's calligraphy is worth collecting depends on how you define "worth collecting". If your purpose is to increase investment, then you can choose to collect his calligraphy works, because his works have certain artistic value and market value. If your purpose is for your own appreciation and hobbies, then you can also choose to collect his calligraphy works, because his works have certain artistry and beauty.
In a word, Gao Rongchang's calligraphy works have certain artistic value and market value. If you like his calligraphy style, collecting his works is a good choice.