The first thing to say is that I would call this an –ing form rather than a ‘gerund’. This is because the term ‘gerund’ is usually taken to refer to a verbal noun (a noun describing an activity that is formed from a verb), such as swimming or painting:

Swimming is one of the best forms of exercise.

Four years ago, Susan took up painting.

What you refer to in your question is something broader; simply a verb in the –ing form:

It is one of the very few occasions in my life when I can recall telling a deliberate lie.

They admitted stealing millions of dollars in rare books and art from Transylvania University.

The second thing to say is that this is a pretty common pattern for English verbs. The most comprehensive treatment I know of English verb patterns is a book called Grammar Patterns: Verbs, which is currently out of print. As the title suggests, it covers all the patterns followed by the main verbs of English and specifies which verbs take which pattern. It lists over 130 verbs that can be followed by an –ing form, including such common verbs as start, stop, love, hate, remember and need:

Then I started going out with this guy.

I love meeting new people and knowing that no two days are the same.

The following jobs need doing.

The verb in the example you give is a phrasal verb: many of these can also be followed by a verb in the –ing form.

To rain hard means the same as to rain heavily, so both explanations of this idiom are correct. In fact, if you look at the entry for rain in the Macmillan English Dictionary, you will see that both adverbs are highlighted as collocates of the verb:

rain heavily / hard (= to rain a lot)

According to the corpus, the two collocates are more or less equal in frequency, although heavily is used slightly more frequently than hard.

Collocation is one of the most difficult areas for learners of a language to master, but you can get a lot of help from your Dictionary. For example, to stay with the weather theme, if you look at the entry for frost you will see that the typical collocates include sharp, hard and heavy. Significant collocates are always highlighted in the entries, and many entries have additional boxes of collocates for particular senses. The entry for the verb fuel, for example, has a list of things that can typically be fuelled, such as debate and fears. The Macmillan Collocations Dictionary, which will be published later this year, will give you much richer and more detailed information on collocation in English.

The quick answer is no: you can say hyphen or dash, but not minus.

I thought it might be interesting to run through some of the other symbols used in email and web addresses.

Email addresses first:
. is ‘dot’
@ is ‘at‘
_ is ‘underscore’.

In URLs (‘you are els’), / is ‘forward slash’ or, more usually these days, just ‘slash’, while www is the very cumbersome ‘double you, double you, double you’. I seem to recall that there was an attempt some time ago to get people to say ‘three double yous’, but it never caught on. Abbreviations such as org, co and com are pronounced as words. As for country names, these are generally spelled out (.uk – ‘you kay’, .fr – ‘eff are’) although some can be said as words (for example, .it can be said ‘it’).