Per @Pheonix2105's answer, in some contexts there might be an implied to [do something to/with] after the word "left". But I don't see that as particularly relevant to the general case. For example...
1: Tom has 10 apples. He gives 3 to Dick. How many are left?
We neither know nor care whether Tom's 7 apples are "to eat", "to give to Harry", or whatever. All that matters is they're remaining (not given away, eaten, or otherwise removed from the original 10 apples).
In such contexts, left is simply an "adjectival" use of the past participle of the verb to leave...
2: Tom has 10 apples. Dick takes 3. How many does he leave?
Note that the last sentence there could just as well have been "How many does this leave?" - it's not really important whether we think of Dick as the "subject" who "leaves" (doesn't take) the leftover apples. If we use this, it's just a demonstrative pronoun meaning this action which has just been described.
If you're interested, see here for some interesting information about "Participle Adjectives" (such as "interested" and "interesting" as used earlier in this sentence).
Note that there are certain limitations on how adjectival left = remaining can be used...
3: Tom sold 3 of his 10 tickets. How many are left unsold? (either left or unsold could be omitted)
4: There are 7 left
5: There are 7 unsold
6: There are 7 tickets left
7: There are 7 tickets unsold
8: There are 7 unsold tickets
9: *There are 7 left tickets (idiomatically unacceptable)
...where #9 is "unacceptable" even though the basic construction is okay with alternatives which are "synonymous" in context (such as unsold, remaining). Because we're more used to adjectival left before a noun meaning "levo" (on the left, not the right side), we would normally express #9 as...
10: There are 7 leftover tickets (or left-over)
All three of those options could be used in that situation:
that post is discussing ...
that post discusses ...
Both of these are pretty neutral present-tense statements. Since for the verb "discuss" there isn't a lot of meaningful difference between the simple and continuative tenses, they basically mean the same thing.
that post is to discuss ...
Using "to be" + infinitive here could mean one of two things:
- You are saying that the post is or was intended to discuss something. (a present tense indicating intention)
- You are saying that the post is expected to discuss something in the future. (an implicit future tense)
Since in the context you used it, the post was already made, the future meaning is unlikely, so most would interpret it to mean that you're talking about the intended purpose of the post (which seems like it makes sense in your context, so if that's what you intended, you're good).
Best Answer
The better choice is "by either". So,
"... by either [a gerund] or [the to-infinitive]..."
Another option is "either [by a gerund] or [by the to-infinitive]", but that uses more words.
I have put braces in the two examples to highlight the structure of the either-or.