To vote for a witness, you simply click here: https://steemit.com/~witnesses

You can vote for as many as 30 witnesses but only the top 50 witnesses are visible on the link i showed you. "steemgigs" is not top 50 yet. it is like 59 th, so you wont see it on the list but there is a box just below the list to vote for other witnesses that aren't top 50 yet, so you can just type in "steemgigs" in the box and vote. Witnesses simply run the servers that hold the steem blockchain, thus, they have to be reputable too or creating value for steem in many ways