It seems you forgot to add the calls to cat in your .gitlab-ci.yml file.

You should have something like that:

script:
    - mvn $MAVEN_CLI_OPTS test
    - cat target/site/jacoco/index.html

That being said, I don't think this is the best way of doing this, as you need to pollute your output with raw HTML in order to retreive the desired coverage value.

I would recommend using the method described in this pull request instead: https://github.com/jacoco/jacoco/pull/488

• Keep the jacoco parts in your build.xml

• Use this awk instruction to print the correct code coverage total:

  awk -F"," '{ instructions += $4 + $5; covered += $5 } END { print covered, "/", 
  instructions, "instructions covered"; print 100*covered/instructions, "% 
  covered" }' target/site/jacoco/jacoco.csv
  

• Replace the Gitlab CI regexp with what the instruction returns: \d+.\d+ \% covered

Edit:

As of Gitlab 8.17, you can define the regexp directly inside the .gitlab-ci.yml file, as stated in the documentation.

It may seem superfluous, but if this regexp is now part of your repository history, you can change it alongside the other tools used to compute it.

I did it that way:

# .gitlab-ci.yml
image: python:3.9

stages:
  - tests
  - min-coverage

variables:
  MIN_COVERAGE: 80  # set the threshold at 80%

unit-test:
  stage: tests
  before_script:
    - pip install pytest pytest-cov
  script:
    - coverage run -m pytest
    - coverage json
    - coverage report -m
  artifacts:
    paths:
      - coverage.json
  coverage: '/TOTAL.+ ([0-9]{1,3}%)/'

min-coverage-test:
  stage: min-coverage
  before_script:
    - apt-get update && apt-get install jq -y
  script:
    - jsonStr="CI_PROJECT_DIR/coverage.json)"
    - coverage="$(echo $jsonStr | jq .totals.percent_covered)"
    - if (( $(echo "$coverage < $MIN_COVERAGE" |bc -l) )) ; then echo "$coverage% of code coverage below threshold of $MIN_COVERAGE%" && exit 1 ; else exit 0 ; fi

Solved the issue by changing image name from gcc to ubuntu 16.04, the working yml will look like this

# use the official gcc image, based on debian
# can use verions as well, like gcc:5.2
# see https://hub.docker.com/_/gcc/
image: ubuntu:16.04

build:
  stage: build
  # instead of calling g++ directly you can also use some build toolkit like make
  # install the necessary build tools when needed
  before_script: 
    - apt update && apt -y install make autoconf gcc g++
  script: 
    - g++ -Wall --coverage -fprofile-arcs -ftest-coverage helloworld.cpp -o mybinary
    - ls
  artifacts:
    paths:
      - mybinary
  # depending on your build setup it's most likely a good idea to cache outputs to reduce the build time
  # cache:
  #   paths:
  #     - "*.o"

# run tests using the binary built before
test:
  stage: test
  script:
    - bash runmytests.sh

coverage:
  stage: deploy
  before_script:
    - apt-get -qq update && apt-get -qq install -y make autoconf gcc g++ gcovr ggcov lcov
  script:
    - ls
    - g++ -Wall --coverage -fprofile-arcs -ftest-coverage helloworld.cpp -o mybinary
    - ./mybinary
    - ls
    - gcov helloworld.cpp
    - lcov --directory . --capture --output-file coverage.info
    - lcov --list coverage.info