A nice little script for converting gradle dependencies from some external source (ahem jcenter) to a local maven repository.

missing-deps-to-local.js

// Based on info from this post https://brightinventions.pl/blog/migrating-away-from-bintray-jcenter-when-there-is-no-alternative-repository/#setting-up-a-local-maven-repository

/**
 * How to use this tool:
 * 1. comment out `jcenter()` in your build.gradle
 * 2. run `./gradlew :app:dependencies > deps.log`
 * 3. mkdir `local-maven`
 * 4. add `maven { url = uri("${rootProject.projectDir}/local-maven") }`
 * 	 right before the commented-out `jcenter()` calls in your build.gradle.
 * 5. run `node missing-deps-to-local.js ./local-maven < deps.log`
 * 6. run `./gradlew :app:dependencies > deps.log`
 * 7. inspect `deps.log`. If there are still `FAILED` deps, do 5 & 6 again until all have been copied over.
 */

const path = require('path')
const fs = require('fs')
const gradleCache = `${process.env.HOME}/.gradle/caches/modules-2/files-2.1/`

const dest = process.argv[2]
console.log(`Copying into ${dest}`)
if (!fs.existsSync(dest) || !fs.statSync(dest).isDirectory()) {
	throw new Error(`${dest} doesn't exist or isn't a directory`)
}

const parseLine = (line) => {
	const parts = line.split(/\s+/)
		// The format reported by :app:dependencies has a lot of `|   |   +----` ascii art.
		.filter(part => part !== '|' && !part.match(/^[+\\]-+$/));
	if (parts[parts.length - 1] != 'FAILED') {
		throw new Error(`not failed - how did we get here: ${line}`)
	}
	let [org, package, version] = parts[0].split(':')
	if (!org || !package || !version) {
		throw new Error(`Unexpected dependency format ${parts[0]}`)
	}
	// If it's of the format com.some:thing:1.2.0 -> 1.3.0, we want to use 1.3.0 as the version
	if (parts[1] === '->') {
		version = parts[2]
	}
	return {parts, org, package, version}
}

const copyDep = ({parts, org, package, version}) => {
	const cacheBase = path.join(gradleCache, org, package, version)
	if (!fs.existsSync(cacheBase)) {
		throw new Error(`Dep not cached: ${dep.join(':')}, not found at ${cacheBase} - ${parts.join('~')}`)
	}
	const mavenDest = path.join(dest, ...org.split('.'), package, version)
	if (!fs.existsSync(mavenDest)) {
		fs.mkdirSync(mavenDest, {recursive: true})
	}
	fs.readdirSync(cacheBase).forEach(hash => {
		fs.readdirSync(path.join(cacheBase, hash)).forEach(name => {
			console.log(`${cacheBase}/${hash}/${name} -> ${mavenDest}/${name}`)
			fs.copyFileSync(path.join(cacheBase, hash, name), path.join(mavenDest, name))
		})
	})
}

const getStdin = (done) => {
	var content = '';
	process.stdin.resume();
	process.stdin.on('data', (buf) => { content += buf.toString(); });
	process.stdin.on('end', () => done(content));
}

getStdin(text => {
	const missing = text.split('\n').filter(line => line.endsWith('FAILED')).map(parseLine)
	missing.map(copyDep)
	if (!missing.length) {
		console.log(`All done! No missing dependencies reported.`)
	}
})

NOTE: if you get errors about a missing .jar, but you have an .aar, you might be able to use mvn to install the .aar to make things happy.
For example:

mvn install:install-file -Dfile=~/.gradle/caches/modules-2/files-2.1/tools.fastlane/screengrab/0.5.6/screengrab-0.5.6.aar -DgroupId=tools.fastlane -DartifactId=screengrab -Dversion=0.5.6 -Dpackaging=aar -DlocalRepositoryPath=local-maven