httpsRequestGuzzle is just a helper function for calling Guzzle. You don't have to use it or override it, but you should be using Guzzle. The gateway already has a working example of using Guzzle directly to call Quickpay API; simply copy it and adjust the data being sent.

mc_amount2 and period2 are used if there's a different initial payment amount and duration in the plan. mc_amount1 and period1 are always the initial payment and duration. mc_amount3 and period3 are always the recurring amount and duration. For recurring subscriptions you typically won't use mc_amount2 and period2 since their values will get pushed to mc_amount3 and period3, but if you had a single payment plan with different initial amount and duration then it's important to have access to amount1 and amount2 for example.