"opening" is definitely the present participle of "open".
Just a note about the gerund in English:
Traditional grammar made a distinction within -ing forms between
present participles and gerunds, a distinction that is not observed in
such modern, linguistically informed grammars as A Comprehensive
Grammar of the English Language and The Cambridge Grammar of the
The distinction between gerund and present participles is not
recognised in modern reference grammars, since many uses are
Distinction from other uses of the -ing form
Having shown that the term "gerund" in English is not at all uncontroversial in its use, I'll give my opinion of different ways it can be identified.
1) Elision of preposition "by" before the verb phrase "opening duty-free shops"
(often with verbal noun) Indicating the means of achieving
2) Seeing "opening duty-free shops" as an adverbial phrase or clause which modifies "making hundreds of millions of dollars".
3) Opening duty-free shops is a gerund clause which is adverbial.
Roles of "gerund" clauses in a sentence:
Adverbial: He walks the
streets eating cakes.
Roles of "gerund" clauses in a sentence
I'm not sure exactly what it is, but I can see it being identified in different ways.