Herd, 27, capped four times by Australia, was a free agent after cancelling a move to Perth Glory.
Nouble, 25, spent last season at Chinese side Tianjin Quanjian.
"In Frank, we've signed a strong, powerful and determined striker. Chris's versatility will be a big boost for us," said boss Justin Edinburgh.
Find all the latest football transfers on our dedicated page.

A survey of 2,750 11- to 18-year-olds found one in 10 admitted checking their mobile phones for notifications at least 10 times a night.
The poll was carried out by Digital Awareness UK and the Headmasters' and Headmistresses' Conference.
The organisations warn night-time usage of mobiles means pupils are coming to school tired and unable to concentrate.
They recommend having a "digital detox" and putting mobile devices away for 90 minutes before lights-out or keeping them out of the bedroom.
Of the 45% of survey respondents who check their phones when they should be sleeping, almost all (94%) are on social media - with a tenth saying they would feel stressed about missing out if they did not check their device before going to sleep.
Of this group, 75% are listening to music and over half (57%) are watching films.
A third (32%) of these youngsters say their parents are not aware that they check their mobile device after going to bed.
The findings also show:
Charlotte Robertson, DAUK co-founder, said: "One of the biggest topics around at the moment is excessive social media consumption and how it is affecting our physical and emotional wellbeing.
"A lot of them [children] are waking up sometimes with over 100 notifications from conversations that have happened overnight.
"They want to be that person that is responding at 01:00, and seen to be quite cool, to make sure they catch the joke - it's a huge driver, that anxiety of wanting to know what's happened."
HMC chairman Mike Buchanan said: "The data suggests those who do check their phones, they're mostly driven by not wishing to miss out.
"Clearly there are some times when children are not concentrating because they are tired, and that has an obvious impact on their ability to keep up with what's going on - there is a desire to stay within the group.
"It's not that this [technology] is all horrible and terrible and that we should all be wringing our hands.
"It is more a case that here's the reality, let's use it and try to influence the use of technology in a positive way."

Erfan Ali was just six years old when he was attacked by his neighbour's Rhodesian Ridgeback called Tyson.
It jumped through a broken wooden fence before attacking the boy in his family's garden, in 2011.
The dog's owner Kevin Large was jailed in 2012 after he admitted failing to control the animal.
Now Erfan's family has won a payout after launching civil action against Large's mother - for not properly maintaining the garden fence between the neighbours in Gabalfa.
Michele Skinner, 52, was ordered to pay the compensation by a judge at the Cardiff Civil Justice Centre.
The boy's legal team claimed damages because Erfan, now 11, will have facial scars for the rest of his life and because of the psychological trauma he suffered.
Erfan's dad Dilwar Ali , 49, said: "This action was never about the money but the principle.
"People need to be able to feel safe in their neighbourhoods without fear of attack from dogs.
"Now hopefully this award will act as a deterrent to unruly dog owners and afford some protection to residents in similar situations."
The family's lawyer said it could lead to many more compensation cases.
Erfan needed emergency surgery at the specialist Welsh Centre for Burns and Plastic Surgery at Morriston Hospital in Swansea.
Since then he has had multiple skin grafts and will need further operations well into his teenage years.
Gillingham have signed former Aston Villa midfielder Chris Herd until the end of the season and striker Frank Nouble on a short-term deal.
[NEXT_CONCEPT]
Almost half (45%) of young people are checking their mobile phones after they have gone to bed, a poll suggests.
[NEXT_CONCEPT]
A Cardiff boy who was scarred for life after being savaged by his neighbour's dog has been awarded £70,000 compensation.